I added the ITUNESADVISORY tag from the Extended Tags window but even though I entered the values 1 or 2 depending on the song, the C for Clean or E for Explicit doesn't appear when I play the song on the Apple Music app on my PC. Please help!
You can try to add this C or E in your Apple Music App (if possible) and then load the same song in Mp3tag.
What fields and content can you see in Mp3tag if you press Alt + T on this song?
Or you can load an m4a file in Mp3tag where this C or E already appears correctly in your Apple Music App and compare it.
Perhaps you need to manually "refresh" or "re-read" the change made in Mp3tag in the ITUNESADVISORY field of your song in your Apple Music App?
(I don't use this App, so I can't tell you exactly where this could be done.)
For a short test, you could rename your song slightly and check if the C or E will be displayed correctly for this "new" song.
It seems to be an issue with Apple Music on Windows, which doesn't show the C for Clean if ITUNESADVISORY is set to 2.
It's showing fine in Apple Music on macOS and in latest iTunes 12.13.7.1 on Windows.