iMessage login problem

Do you get the same error message when logging into your account? At https://appleid.apple.com

Also try updating the iOS software to 12.3

Check the date & Time is correct on your iPhone, that can cause some issues.

Restart your iPhone

If you've tried all of these and you're still getting that error message, I would say contact support and check everything is okay with your Apple ID.

https://getsupport.com.au

Messages - If you can‘t sign in to your iMessage account or FaceTime on your Mac

Message activation error

Have someone else attempt to sign in using their Apple ID. If they can successfully sign in and use Messages/FaceTime, it may be your Apple ID. Someone I know had this happen and I was able to sign in using my Apple ID. After a call to Apple Support and then being elevated to a tier 2 support person, it turned out it was a problem with their Apple ID. It took the software engineers 3-4 days to fix the problem. When calling Apple, make sure you tell them it is an Apple ID problem so you don't get charged.

You can also try signing in on another device/computer. There have been a couple of posts where people's device/computer was blocked. You will need to contact Apple for that problem also.

There is a contact link.

Apple ID Support

Apple ID Support (2) 

Apple Support by e-mail or chat         

Apple Support using Twitter

Or fill out and submit this form.