iPad Pro 12.9" (6th Gen)
It's not possible to send and receive iMessages
Solution 1 of 4: There are problems activating iMessages
You can send iMessages to phone numbers or email addresses if the recipient has a device which supports iMessage. To set up your tablet for iMessaging, you need to activate your Apple ID.
Press Settings.
Press Messages.
Press the indicator next to "iMessage" to turn on the function.
Slide your finger upwards starting from the bottom of the screen to return to the home screen.