Large Vehicle Mag Calibrate Failed to Execute

#### Issue details
Hi
I have a rover that weighs a few hundred kgs so it is impossible to calibrate the compass by rotating the rover.
I an trying to calibrate the compass by clicking the Large Vehicle Mag Cal button when I enter the heading an error displays saying command failed to execute.
It has a good RTK GPS lock.

#### Version
Mission PLanner 1.3.75 build 1.3.7883.26333
ArduRover V4.0.0

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ X] Rover
[ ] Sub

#### Airframe type
Ground Rver

#### Hardware type
Pixhawk 4

#### Logs

Start by reproducing the failure from the Large Vehicle Mag Cal button using Mission Planner 1.3.75 with an ArduRover 4.0.0 rover and Pixhawk 4, noting the entered heading and displayed command error. Trace the button's command path and compare it with the available RTK GPS state. Done means large-vehicle compass calibration executes successfully or reports a specific actionable failure.

Written by the indexing model from the issue text.