Hello.  Would anyone in the know be able to help?  I am pressing the NAV/GPS button on the cockpit beside the Garmins to get the autopilot coupled to the GPS but it will not shift out of NAV mode.  I have the Flightplan loaded and have pressed the CDI button.   Any thoughts what I am doing wrong?  X-plane 11.2 and default Cessna 172 floatplane.  Thanks.

Assuming you are using a default aircraft, it should be enough to push the CDI button on your GPS.

I've never had to do it differently and it has always worked for me.

My best guess is that you have a plugin that interferes. Try disabling all plugins, load a default aircraft and see if it helps.