Notification badge will not clear no matter what I tap

This is the launcher badge desyncing from the app. Clear the app cache, then long press the icon and toggle the badge/dot setting off and on in your launcher settings. If you use a Samsung or a custom launcher, that badge count is stored separately from the app.

Nine times out of ten there is one unread in Message Requests. Open that folder specifically, not just the main inbox.

Restarting the phone entirely did it for me when nothing else worked.