iMessage activation an error occurred during activation. how can I solve it?

See If you get an error when trying to activate iMessage or FaceTime  https://support.apple.com/en-us/HT201422. Use these support options if you need to.

Get a call from Apple support to help solve your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362

Apple ID Support https://support.apple.com/apple-id

Apple ID account security for country-specific and region-specific help

 https://support.apple.com/en-us/HT204169

Device or computer?

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

Message activation error.