Cannot load mission route

I have calibrated my airspeed, run the autotune setup and my plane loiters perfectly.

I tried to load a simple route in Mission Planner but get the error message “Upload failed. Mission was rejected by the mav. Item had a bad option wp#1 MAV_MISSION_INV”

Attached is a screen shot of the 4th attempt.

What am I doing wrong?

Update - changing the altitude control from Terrain to Relative or Absolute solved the problem and the mission uploads fine.

Why won’t it accept Terrain as this is the most sensible one to use.