This error has been discussed and answered many times on the forum, and it's caused by the fact you either launched the installer when FSX was still running, or it wasn't entirely closed.
A Windows restart wil surely fix this.
If it doesn't goes away after a Windows restart, it's caused by your antivirus which has mistakenly quarantined the Couatl.exe file. You should install with the antivirus disabled, and configure it to exclude the following files from scanning before launching FSX:
YourFSXFolder\bglmanx.dll
YourFSXFolder\fsdreamteam\couatl\Couatl.exe
