Well this does not work because the installing user was MSFS2020. I found a solution for that problem, your installer does not work correct in my case, please have a look at the attached log.
The solution in my case was to make a symbolic link between: C:\Users\MSFS2020\AppData\Roaming\Microsoft Flight Simulator\UserCfg.opt from M:\WpSystem\S-1-5-21-3480065646-2766658366-1087699166-1001\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache\UserCfg.opt
where my real installation is placed.
2020-12-16 09:53:20.716 Log opened. (Time zone: UTC+01:00)
2020-12-16 09:53:20.716 Setup version: Inno Setup version 6.0.5 (u)
2020-12-16 09:53:20.716 Original Setup EXE: E:\MSFS2020\Payware\lszh_msfs_setup.exe
2020-12-16 09:53:20.716 Setup command line: /SL5="$F06C4,304188734,892416,E:\MSFS2020\Payware\lszh_msfs_setup.exe" /log=log.txt
2020-12-16 09:53:20.716 Windows version: 10.0.19041 (NT platform: Yes)
2020-12-16 09:53:20.716 64-bit Windows: Yes
2020-12-16 09:53:20.716 Processor architecture: x64
2020-12-16 09:53:20.716 User privileges: Administrative
2020-12-16 09:53:20.723 Administrative install mode: Yes
2020-12-16 09:53:20.723 Install mode root key: HKEY_LOCAL_MACHINE
2020-12-16 09:53:20.723 64-bit install mode: No
2020-12-16 09:53:20.726 Created temporary directory: C:\Users\MSFS2020\AppData\Local\Temp\is-A1HSB.tmp
2020-12-16 09:53:25.987 Previous FSDT root set in the registry at:
2020-12-16 09:53:36.533 MSFS not found in current user account, looking for other accounts now
2020-12-16 09:53:36.533 Searching for MSFS in C:\Users\All Users\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.533 MSFS not found in C:\Users\All Users\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.533 Searching for MSFS in C:\Users\Default\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.533 MSFS not found in C:\Users\Default\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.533 Searching for MSFS in C:\Users\Default User\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.533 MSFS not found in C:\Users\Default User\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.533 Searching for MSFS in C:\Users\Denise\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.533 MSFS not found in C:\Users\Denise\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.533 Searching for MSFS in C:\Users\Jens\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.533 MSFS found in another user account C:\Users\Jens\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.533 MSFS not found in current user account, looking for other accounts now
2020-12-16 09:53:36.533 Searching for MSFS in C:\Users\All Users\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.533 MSFS not found in C:\Users\All Users\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.533 Searching for MSFS in C:\Users\Default\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.533 MSFS not found in C:\Users\Default\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.533 Searching for MSFS in C:\Users\Default User\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.533 MSFS not found in C:\Users\Default User\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.533 Searching for MSFS in C:\Users\Denise\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.534 MSFS not found in C:\Users\Denise\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.534 Searching for MSFS in C:\Users\Jens\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.534 MSFS not found in C:\Users\Jens\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.534 Searching for MSFS in C:\Users\Jens Habermann\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.534 MSFS not found in C:\Users\Jens Habermann\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.534 Searching for MSFS in C:\Users\MSFS2020\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.534 MSFS found in another user account C:\Users\MSFS2020\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.534 C:\Users\Jens\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache\UserCfg.opt not found
2020-12-16 09:53:36.534 C:\Users\MSFS2020\AppData\Roaming\Microsoft Flight Simulator\UserCfg.opt not found
2020-12-16 09:53:36.534 MSFS not found in current user account, looking for other accounts now
2020-12-16 09:53:36.534 Searching for MSFS in C:\Users\All Users\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.534 MSFS not found in C:\Users\All Users\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.534 Searching for MSFS in C:\Users\Default\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.534 MSFS not found in C:\Users\Default\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.534 Searching for MSFS in C:\Users\Default User\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.534 MSFS not found in C:\Users\Default User\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.534 Searching for MSFS in C:\Users\Denise\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.534 MSFS not found in C:\Users\Denise\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.534 Searching for MSFS in C:\Users\Jens\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.534 MSFS found in another user account C:\Users\Jens\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache
2020-12-16 09:53:36.534 MSFS not found in current user account, looking for other accounts now
2020-12-16 09:53:36.534 Searching for MSFS in C:\Users\All Users\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.534 MSFS not found in C:\Users\All Users\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.534 Searching for MSFS in C:\Users\Default\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.534 MSFS not found in C:\Users\Default\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.534 Searching for MSFS in C:\Users\Default User\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.534 MSFS not found in C:\Users\Default User\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.534 Searching for MSFS in C:\Users\Denise\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.535 MSFS not found in C:\Users\Denise\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.535 Searching for MSFS in C:\Users\Jens\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.535 MSFS not found in C:\Users\Jens\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.535 Searching for MSFS in C:\Users\Jens Habermann\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.535 MSFS not found in C:\Users\Jens Habermann\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.535 Searching for MSFS in C:\Users\MSFS2020\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.535 MSFS found in another user account C:\Users\MSFS2020\AppData\Roaming\Microsoft Flight Simulator
2020-12-16 09:53:36.535 C:\Users\Jens\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalCache\UserCfg.opt not found
2020-12-16 09:53:36.535 C:\Users\MSFS2020\AppData\Roaming\Microsoft Flight Simulator\UserCfg.opt not found
2020-12-16 09:53:36.535 MS Flight Simulator not found, the installation cannot continue
2020-12-16 09:53:36.537 PrepareToInstall failed: MS Flight Simulator not found, the installation cannot continue
2020-12-16 09:53:36.537 Need to restart Windows? No
2020-12-16 09:53:41.871 Deinitializing Setup.
2020-12-16 09:53:41.884 Log closed.
Kind regards Jens