Can't delete music from iOS device in iTunes
This is likely because iTunes is automatically syncing your music.

When you automatically sync music, you manage what songs you have on your device by going to the music tab, and selecting which playlists/artists/genres/albums to sync or not sync.
The fix:
To manually manage music, click the Summary tab, scroll to close to the bottom, and in the Options area, there will be an option to Manually manage music and videos. Select this option, and you should be able to delete songs and such from the &quot;On this iPhone&quot; tab.