QGC doesn't let me arm

Despite having all categories green in the ‘summary’ section, when I try arming it gives me the following message.
Help anyone?
Thanks in advance

There is a major difference between (a) setting up the copter/plane, and (b) passing preflight checks. The preflight checks cover about a dozen things, primarily instantaneous sensor readings. The goal of the preflight checks is to figure out if the plane/copter is ready to fly in that instant, not whether it is set up correctly in general. There should be more messages - airspeed, compass, battery, GPS, etc. that should help you to figure out which specific sensor/subsystem is causing the fail. A typical problem is e.g. not to have the pitot cover in place when turning on the system, which will throw high airspeed warnings.

A basic startup sequence for an airframe is:
RC transmitter On
Airframe on level ground
View of the sky
Pitot cover on
Not near big block of iron
Plug in battery
Don’t wiggle airframe for several seconds
Set Mode: Stabilize
Kill switch off
Airframe arm: Arming switch hold for ~2 secs - confirm blink pattern change and servo activity
Pitot cover remove
ESC arm: RC throttle down and right for ~2 secs
Device ready