I have a Mavic Pro that is about 2 years old.  It has never been crashed/dropped.  I am having problems with getting the compass to calibrate.  The remote/DJI Go 4 app screen shows "? Calibrate Error" (see pic).  I have reset/calibrated the IMU with success, I have calibrated the compass multiple times with success, I have changed locations, and the error message will not go away.  Any help with this would be great.  Thanks  

Using a CFixer http://cfixer.com/ might help to clear the problem up. The cause isn't completely clear from the details you've given.

Was the UAV stored in a hot or cold area for any length of time? This could lead to some of the circuitry to cause a malfunction.

Hi, we're sorry for the inconvenience that it caused. You may try to refresh the firmware version into the latest and re-calibrate the compass to see if there will be changes. The DJI Assistant 2 can be used to refresh the firmware which can be downloaded using this link: https://www.dji.com/mavic/info#downloads. If the problem persists, you may try to re-install the application or change a mobile device to see if there will be changes. If the situation will not change, please contact our support using the link below for a further evaluation of the unit.

www.dji.com/support

Hello, download/refresh the firmware version into the latest and re-calibrate the compass. Did you store your MAVIC PRO near a magnetic field?

Ensure to calibrate away from any large metal objects