I have zero experience with this with chickens, but I have tried this with cats before. For some reason, being on a leash is just SO foreign to cats that they freak out when they feel it preventing them from going in the direction they want to go. I'd assume it's the same for chickens. Your chicken just may have no clue what's going on to prevent her from moving freely, and she just freaks out. I have no clue if there is a good way to leash train a chicken, but maybe try putting her on it and then doing things that calm her. Maybe bring out her favorite snack and toss it in a few different directions while she's on the leash. If she is a chicken that likes to be pet or held, maybe do that stuff with the leash on too and see if she calms down.
Again I have no experience with this but I wish you the best of luck. Keep us updated if you make progress! And if she just hates it too much and won't calm down, it might be best to just accept that she won't be a leash chicken.