Drone unable to arm (No error message)

Hi all, I am currently using APM2.8 at Mission planner firmware v3.1.5 for optical flow purposes. However I could not arm my drone at all and I keep getting single red flashing lights only. There is no error messages at all under message tab.

Please help thanks in advance!

Open the Pre-arm Check tab, set it off and try, then turn it on one item at a time and you will find why there is no arming