Thanks for stopping by the Apple Support Communities. From your post, I understand that the on-screen keyboard does not appear as expected in the Messages app. I'd be happy to help!
If you still have trouble with the keyboard after those two steps, check for iOS updates: Update your iPhone, iPad, or iPod touch - Apple Support, and if you're using a third party keyboard, check for updates on that, too (which you can find in the App Store > Updates).