What to do if your kitten can't poop
Constipation can be quite uncomfortable for kittens, and could be caused by:
Formula that is too concentrated
Not toileting frequently enough
Intestinal obstructions (from eating foreign objects that block the stomach/intestines)
Severe intestinal parasites
Making sure the kitten is hydrated should be priority (oral electrolytes and subcutaneous fluids). Applying a warm heat pack to their abdomen and gently massaging them, or small warm water/microlax enemas may help relieve some of the constipation until the underlying cause is treated. If the kitten does not pass faeces after treatment at home, you must take them to a Vet who can provide further assistance.