Check that:
- The drive the community folder is formatted in NTFS, which should be the default for internal drives.
- Both GSX packages (3 in MSFS 2020) in the Community folder are NOT real folders (=missing the small arrow on their icon). If they are real folders, close the FSDT Installer, REMOVE them, restart the FSDT installer and RELINK should work.
- Be sure you have admin rights with you star the FSDT Installer, it should be the default anyway.