First, before we get started diagnosing any issues that you may be having, make sure that you've properly installed the latest mod installer.
How To Download And Install New Custom Songs On Beat Saber
While you're at it, also verify that the "Playlists" folder was created too. This was fixed so it doesn't break if it's not there, but a folder should have been creating by the ModInstaller, songDownlader or SyncSaber itself.
Default Steam Location
Default Oculus Location
It is NOT the plugins folder that is in your Beat Saber_Data\Plugins folder: