I have done some digging and reading and have found why the plane will not arm. It is to do with
Prearm EKF Yaw Inconsistent. There has been a lot of other users having this problem see this link
EKF2 Yaw Inconsistent and it has been going on for a year now. You can recalibrate external compass, use internal, use both compass, but the result is the same. But as one contrbutor found if you turn the plane to roughly North, the error disappears, but as yu turn from 0 degrees toward 180 degree, the error increases up to 180 degree. Then continuing around to North, the error gradually reduces bac toward zero. So this is the reason my plane will not arm via stick. I am running Plane 4.06. I see there is an update ot now to ver 4.08, but I doubt if the problem is fixed.