So I have an Iris+ and was putting drift mode on in position 1 of which the other 2 were loiter and auto. When I went to fly it would not arm in drift, wondering why?
I am sorry… but i must say this: RTFM! 
copter.ardupilot.com/wiki/flying … he_motors/
“You can only arm or disarm in Stabilize, ACRO, AltHold and Loiter mode.”