I've run into this same issue. In my case it's connected to the fact that, for OS safety reasons, the Windows account I run FS2020 in is NOT an 'administrator' account, but a 'standard user' account. When I launch the FSUIPC installer, it wants to 'run as administrator' which means it's looking for UserCfg.opt in the %localappdata% folder under C:\Users\Admin\Appdata\Local' and not finding it, which makes sense because the file actually exists under the path 'C:\Users\Capho\Appdata\Local\...'
In the installer file sample posted above, it also references two separate user names ('c:\users\pauld' and 'c:\users\emman') so I suspect the same thing may be happening there.