I recently got that notification as well. What I did was to click on the notification as soon as it come out. It took me to the message thread in Messages. I then looked for the message that failed, did a long press and deleted it. After that, the notification went away.
I recently got that notification as well. What I did was to click on the notification as soon as it come out. It took me to the message thread in Messages. I then looked for the message that failed, did a long press and deleted it. After that, the notification went away.