Did you already make sure, that your photo number is not selected, but your email address is enabled in Settings -> Apps -> Messages -> iMessage -> Send & Receive?
If yes, restart the iPhone and see if you can use iMessage after that. If not, log out of your Apple Account in Settings -> Your Name -> scroll down the screen -> Sign Out.
Restart the iPhone and sign in again and test iMessage after that.
If you are getting specific error messages when trying to activate iMessage, have a look at the troubleshooting in this article:
If you can’t turn on or sign in to iMessage or FaceTime on your iPhone - Apple Support