GPS location keeps changing while it's on table!

When I connect APM 2.6 to Mission Planner using USB cable the Quad location on the map is changing continuously !

It keeps to change around a large area, about 15 m or more around, see the attached image.
It is a large error I think, isn’t it ?
I did load the firmware again and calibrated everything for many times but it is still the same!

How can I solve that ?
Is there any settings to do ?

Go outside. The reason is you don’t have a clear view of the sky. GPS doesn’t work indoors.