As Amykins said, it looks like rickets but with ducks it generally is not enough niacin - Vitamin B3. How old is the duck? Any injuries that you know about?
I would let her swim in water deep enough to float in. Not hot water, not cold water. Lukewarm. This will improve blood circulation, take weight off the legs. If she is a Pekin - white ducks weighing about 3 or 4 kilos, (8 or 9 pounds), she may be more likely to have leg problems, but you can help her.
If you can get B3 vitamins, add 150 mg of the vitamin to 4 litres of drinking water. Use plain niacin capsules. Not timed-release or no-flush, just plain B3.
If you cannot find B3 powder, you can give brewer's yeast - not for baking, use the brewer's yeast, or nutritional yeast. It has B3 and other vitamins in it. Add a tablespoon to her food daily.
You will probably need to add the vitamins for two months at least. Daily time to float in water will help.
I took her to the doctor. She said : the duck has a slipped achilles, and we can't help her..:'(.
You have any idea, how can I help her?
Please help me
Without photos it's hard to see what the problem is. Have you been giving her niacin? Maybe you can try to manipulate the tendon back in place and then get a "duck foot" over it to help it heal, like this: