iPad Pro 12.9 (2018)
It's not possible to send and receive iMessages
Solution 3 of 5: Your iMessage isn't written correctly
You can send iMessages to phone numbers or email addresses if the recipient has a device which supports iMessage. To write and send iMessages, you need to set up your tablet for iMessaging.
Press the message icon.
Press the new message icon.
Press the field next to "To:" and key in the first letters of the recipient's name.
Press the required contact.
Press iMessage and key in the required text.
Press the send icon.
You can also send pictures, video clips, audio files and different effects in your iMessage. Follow the instructions on the screen to send your iMessage with the required content.
Slide your finger upwards starting from the bottom of the screen to return to the home screen.