Message Failed to Send notification

Every time my phone restarts or the battery dies, I get this notification stating a message failed to send. The thing is, the message has been sent a while ago and the history has already been deleted. The number is not in my contacts either. I’ve tried resetting my settings and turning off and on MMS messages. The text wasn’t an iMessage either, just a regular SMS message. I’ve also tried tapping on the notification but nothing happens. I’m unable to update iOS because my phone doesn’t support iOS 13.