SOLVED Message Failed To Send notification
I had a notification on my Home Screen: Messages Failed To Send and I could not get rid of that notification for the life of me. Tried everything from deleting all message threads, the contact, cleared caches plus a plethora of suggestions found here on iPhone support. Nothing worked. FINALLY got it done by going to: Messages>Notifications>then unchecked all the ALERTS.
After HOURS of research, that did it! Hope that helps as I saw this was a problem for many others! You’re welcome.
iPhone XR, iOS 16