The "6dof Tracking is not available" warning should only show temporarily when you initiate a scan.  If this warning does not go away, try changing where you are aiming to make sure that the 3D camera is pointing at objects well within its range and preferably with lots of geometry in the scene (as opposed to pointing just at a flat wall.) 

If this doesn't work, then the Apple ARKit 6dof tracking is not working for some reason. Try shutting down your iPhone or iPad completely and restarting it.  This should reenable the 6dof tracking.