I have run into the same problem and is not allowed to access the internal storage, strangely i am allowed to access the system_ext folder end sub folder.
I have migrated from a Samsung S8 plus to a One Plus 8 pro, the music was on the SD card in the S8plus and PA kept the SD card location after the migration. The One plus does not have a SD card slot so had to move the music on to the internal storage, dumped it from the computer into the music folder, did a full re scan and the music from the SD card was removed form the PA index but the music from the internal storage was not added. Tried to use the library settings and navigate to the music folder, but I was not allowed to access the internal storage.
Also strange is that I by selecting a music file directly I can play it with PA and the entire album from where the tune was from now became a playlist but if I press the playlist it will not play.
NB. I did install the Files by Google as mentioned in this thread, I will also try to reinstall the program from the link (it was installed from Google play) and as a last resort try to run the file access in legacy mode. One thing I did notice reading the entire thread is that most report this problem after a migration and installing/migrating using Google Play.