Compass Calibration goes well but Error Compass Variance during takeoff

We are making a Octo Quad X8 copter with axis length 1600 mm.
We are facing some problems with the compass.
The FC in use is Cuav x7 and the RTK is CUAV C-rtk 9ps.

The calibration goes well but during simple takeoff in stabilize mode, we receive ‘Error Compass Variance’ error.

We tried recalibrating multiple times but the situation is the same.

We also used a holybro rtk, but the situation is similar.

What could be the issue?