I know it is not the answer you want, but you cannot expect such dated software to work on Win11
There are just too many requirements for FS 2004 that are not found on Win11 and compatibility is the only issue that I can think of.
Why are you using FS2004, this program came out in 2003 and FSDT has stopped writing addon's for it long ago, so Addon Manager being installed for FS2004 is a long shot. And without the Addon Manager, you're kind of hooped.
But, computers can be kind of funny.