iMessage isn't activating
i don’t know why my iMessage didn’t activate
iPhone 8 Plus, iOS 16
Did you already check the general troubleshooting mentioned in this related article?
If you can’t turn on or sign in to iMessage or FaceTime on your iPhone - Apple Support
In case this does not work, you will have to contact the official Apple Support and ask them to look into that issue from their side:
You can choose the phone number for your region from this Apple page: