Years ago I tried to send a text message from my iPhone to a friend. The message wasn’t delivered because the friend had meanwhile changed her phone number, but my iPhone still tries (unsuccessfully) to send it, and sometimes (not every time) I restart my phone after shutting it down, I get a “delivery failed” message. It was even carried over from my old physical device to a new one when I used the cloud to clone the old device to a new phone. I can’t find an out tray in the messaging app that I could delete the unsent message from. So is there another way to get rid of these “delivery failed” pop-ups?
Can you edit the message? I think if it gets down to zero characters, iPhone decides it isn’t a message any more. Just a hunch.
Is the conversation still in the Messages app? If it was years ago, you might have to scroll down the list a bit. If so, open the conversation and tap and hold on the message until a menu pops up. Tap “More…” Then tap the trash can icon located at the bottom left. Tap “Delete Message.” If you want to delete the entire conversation, tap “Delete All” at the top left.
You can also go to the main “messages” menus and swipe that one left and click delete.