Often my Mavic Air indicates that there are magnetic interference and I must calibrate the compass, but there is not magnetic interference (I used a magnetometer to measure it) and is very difficult to calibrate the drone to fly. Even making all the process as is indicated sometimes I can´t fly. It's really a problem because I plan to do my work, I travel to the field but there is no way to fly the drone. It happens in different areas, all rural areas were there is no wires of electricity or other kind, no concrete with iron, bridges, etc etc
What can I do?

I sometimes get that problem with my Mavic 2 Zoom, but re-calibrating always sorts it out.  Maybe an obvious observation, but is all your software and firmware for the drone up-to-date?

Hi there. Thank you for reaching out and we're sorry for the trouble. Can you please try to calibrate the aircraft in an open area without any possible interference and check if the error message remains? In addition, you may plug in your DJI Mavic Air aircraft via DJI Assistant 2 software (download the link below) on your PC to update/refresh the firmware. Please keep us posted. Thank you.

https://www.dji.com/pt/downloads/softwares/assistant-dji-2

Sounds like the same issue I have with one of my Mavic 2 drones.
It reports mag interference on almost every startup. But if I see in the menus of Go4, under sensors state, the compass sensors are green and OK, no interference detected.
And I also check to see if the drone symbol on the map is pointing the same direction as the drone itself on the ground.
If I take off without calibrating, it flies perfectly fine.
So I have concluded that it is a software glitch that triggers the warning, especially since my other drones do not give that message on exactly the same place.

But when I get that message, I always do the checks described above, because one day it may be a real warning!