LOL. She's just being chicken.
There's a reason we call it that. Chickens are fearful creatures. She apparently is not impressed with the outside environment.
Slowly introducing her to the outdoors is one method, giving her more time to adjust.
Another method is to leave her out for a couple of hours and simply let her adjust. She'll get used to it and start moving around.
But...obviously watch her closely to make sure she is not exhibiting illness. If she is totally fine and normal in the brooder, but lays down outside only, then I'd say it is environment adjustment.
LofMc