Unable to enable iMessage on iPhone 6s

You might additionally see if anything in this article could be helpful:

If you can’t turn on or sign in to iMessage or FaceTime on your iPhone - Apple Support

We need more information if we are to try to help you. Please let us know the following.

• Is this a new problem? Were you able to use iMessage previously on this phone?
• Are you able to open the messaging app?
• Are you able to read messages people have sent to you?
• Are you able to start a new message?

If none of this is possible, then please tell us what happens when you try to do any of this.