1 month old chick has rubber legs
A little more information will be helpful.
How old is she?
Can you post some photos of the position of legs/how she is laying?
Any other symptoms such as lethargy, not eating/drinking?
Was she vaccinated for Marek's?
What does her poop look like?
Some leg issues can be caused by Vitamin B deficiency, especially Vitamin B2(riboflavin), you can add some poultry vitamins to her water or you can use a human B-Complex tablet which has all the Bvitamins (make sure there is no iron in the ingredient list).