See the troubleshooting steps here, If you get an error when trying to activate iMessage or FaceTime - Apple Support
Your phone number will not appear until iMessage is activated. At this time it isn't. You need to follow the troubleshooting steps in the support document that I linked. Make sure that you are using a carrier that is supported by Apple, and that the carrier supports iMessage. To activate, a hidden SMS is sent to the iMessage server in the UK. If you do not have a SMS plan for your phone, or the carrier blocks the sending and receiving of international texts, that could be the cause. Just follow the steps in the support document.
Tried everything in there but nothing worked. My date and time is set to automatic and I am on. iOS 11.3.1. I wonder why it went back trying to activate when it was working fine earlier this month.
If you tried everything, the last step is to contact Apple Support. What did they say when you contacted them.
I’m going to schedule a call soon. Thanks anyways.
Unfortunately, once you follow all of the steps in the support document, only Apple can assist.
imessage activation error