confusing error msg on APM2.x with attempted terrain mission upload

I helped a user today who kept getting a very confusing msg when trying to upload a mission on an APM2.x. They had the terrain altitude option set in Mission Planner and the msg they get when trying to upload a mission didn't given them a clue as to what was wrong.
Can you check if it's an APM2.x and give a better message?
Thanks!

Start by reproducing a terrain-altitude mission upload on an APM2.x and locate the current upload error message in Mission Planner. Done means the message clearly explains the APM2.x limitation or problem when terrain altitude is enabled.

Written by the indexing model from the issue text.