Small_angle is a safety feature for larger quads. I think the default is set to 25°. That means that if the flight controller is at an angle >25° in any orientation, the quad will not arm and spin the motors. It’s designed so that if you’re holding your 5” and accidentally flip your arm switch, the quad will not eat up your fingers, hands, or arms. That’s not really a concern with a tiny whoop.
If the fc isn’t set perfectly to level, it’s easy for the angle to exceed the 25°. Setting “small_angle = 180” (180°) in the CLI (Command Line Interface, the bottom tab on the left of betaflight) means that the fc will arm at any angle, even upside down.