It appears you are using the X-Plane 12.4 beta. There will be an update coming with fixes for 12.4. The error message indicated is a warning that has no bearing on use of the product (just an old dataref being read by the plugin)
Regarding the GTN, on the screen freeze, that is likely an issue with RXP itself.
Regarding the GNS, do any of the other buttons work, or is still always just showing the GNS load screen?