Can't arm quad in betaflight

To test the motors/ESC you should not arm the quadcopter. You check the checkbox on the motors tab saying that everything is safe, then you move the master slider via betaflight software to check that all ESC start spinning at the same time.

Looking at the motors with the quadcopter armed is not particularly helpful due to the way the flight controller algorithms work as the vibrations on the desk/workbench mess things up.