What elevation or feature to use for parking lot & aprons

I am doing take off for the first time, running into a roadblock getting the airplane hanger apron to calculate, and not give me a generic error.  It's not flat, so I didn't want to use "platform" I have tried contours and path and continue to get error codes.

https://photos.app.goo.gl/HSWeY7WVFbwMYybJ7

Hi Burro, look into using Feature Surface and Outlines to determine the extent of your apron and any known levels, then add Contours, Breaks and Points to the Outline if required.