Compass Calb. Error 99

This is my first DIY Drone a S500 With a APM2.8 ( I think, it has internal compass } On setup in comp. calb. I get a error 99 and no dots appear, I would like to use A JMT GPS / Compass. Please help Thanks Dave

Apm2.8 doesn’t have an internal compass.
If your external compass isn’t recognized, have a look here: