Probably an Android 10/11 file access issue. Check Settings > Library > Music Folders and ensure that you have granted access to the specific location where your music is stored (recent versions of Android won't allow you to just select the root of an SD Card for example) and that you have then ticked the relevant folders in the selection list for PA to scan. If that still doesn't it work, look at Settings > Misc > File Access Legacy Mode. If it's on, turn it off, and vice versa