iMessage not delivered and unable to add email on my iPhone

Try tapping on "Use your Apple ID for iMessage" instead of adding a different email address. If you get the same error message, try updating the iOS; it appears you are currently operating your iPhone 7 on iOS 15. If you are looking to add an email address other than the one you use as your Apple Account email, go to Settings --> Apple Account --> Sign in & Security. There you can add an email address (or phone number) that you can then use for iMessage. You'll have to first confirm ownership of the email address (or phone number) you wish to add. These should be good starting points.

Good luck

Your reception is at one bar, which may be an issue if the service cannot reach Apple’s servers for activation. Try going to an area with a stronger signal and re-attempt logging into iMessage. Alternatively you may be able to activate it by connecting to WiFi. Connect to WiFi and see what happens. Is this the only service you are having trouble connecting to, i.e., iMessage? What about FaceTime? Seeing the same trouble with that service too?