I am having the exactly same problem. Pixhawk 2.1, with AC3.4, there was NO problem. loaded AC3.5, "Prearm: RC Roll not confgiured".
after I calibrate RC, this error mesg went away. HOWEVER, when I fly my copter ( quad ), the roll does not work correctly. it led to crashes, I tested several times.
also, I upgraded mission planner to 1.3.49, it is not able to load AC3.5 onto my pixhawk 2.1
I had to use qgroundcontrol to load ac3.5, set the frame using qgroundcontrol. Otherwise, mission planner 1.3.49 keeps saying: prearm: check platform or frame_class.
so I would have to say: AC3.5 is a failure, stay with 3.4, do NOT go 3.5. hopefully these fatal bugs can be fixed as quickly as possible.
my same pixhawk 2.1, NO issue with AC3.4. Sadly becomes a piece of junk with AC3.5, 100% useless now.