GPS numsats 5 (needs 6)

the title basically explains my problem.
When I connect the Mission Planner with my Pixhawk I can arm my drone with my RC transmitter.
After I switch over to guided-mode(doing it with mavros) I can’t arm it anymore. I get the error message:
FCU: PreArm: GPS numsats 5 (needs 6). Now I can’t even arm with the RC transmitter anymore.
Don’t know what the error means. Can anyone help?

I was thinking about putting the drone in a better position. Could it be that it’cs currently in a not so ideal position? It’s indoor,near to the window. Was thinking about putting it outside to try again(when it stops raining), but if that could be a reason, then i don’t now why it arms in the first place.

Ok, I tried it outside, still same problem. I tried to change the minimum expected satellites to 5, but somehow it still expects 6.

It should be outside away from trees and buildings to work correctly. Inside will not work.