The latest version of iTunes — 12.7 — removes the App Store. That’s bad news for folks who like to keep backups of old iOS apps around, but good news for people who have bloat and clutter. But the update also removes all your custom ringtones, so you can’t manage them from your Mac.
Don’t despair. You can still download purchased ringtones, and copy your own tones across from the Mac. It’s just not obvious how to do it any more.