Good afternoon Richard Jones22,
Welcome to the Apple Support Communities!
I understand you are seeing a "Message failed to send" notification every time you restart your iPhone. This is for a message that was sent "298 days ago."
Are these messages stored on your iPhone, or do you have Messages in iCloud enabled? Keep all your messages in iCloud
Have you tried to disable notifications for the Messages app, restart your iPhone, and then enable the setting again?
Change message notifications on iPhone - iPhone User Guide - iOS 12
Your profile shows an iPhone 6s with iOS 12. Is that right?
I would suggest updating your iPhone to iOS 13.2, and then restarting your iPhone.
Update your iPhone, iPad, or iPod touch
If the issue persists, you may need to restore your iPhone to factory settings, and set it up as new.
Restore your iPhone, iPad, or iPod to factory settings
Take care.