You might get more responses by providing details. And just out of curiosity, how is this an iOS 12 issue when a 492-day old message couldn’t haven’t been sent with anything newer than iOS 10.
Taking a stab saying its an iOS12 issue since it popped up yesterday and I have been on iOS12 since the 4th. Its quite possible its not but hey who knows. What is happening is after turning on my iPad I will get a notification saying "Message failed to send" Interestingly enough its not happening on my phone which is also iOS12. I found a few older threads with tips, none seemed to have worked. I tried downgrading to 11.4, updating, and even restoring and it still occurs....
This is old problem and sometimes happens after reboot and so on on iOS 10, iOS 11 as well. Maybe even older iOSes...
Yeah...I've had this happen on occasion after a beta update throughout iOS11...always clears itself up after a hard restart
I started having multiple issues aside from this popping up. It seemed to stem around erasing and restoring. In process of reverting back to 11.4 going to backup to PC and then re attempt beta...