Why is my iMessage is not working

Do a hard reset on your iPhone and then try again. To do a hard reset you need to press the up volume and release, press the down volume and release and then press and hold the power button until the white apple logo appears on your phone. Once you see the apple logo you can release the power button and once the phone comes back on you can try again to activate your iMessage

Hello,

See this:

If you get an error when trying to activate iMessage or FaceTime

Good luck.