She would poop at the beginning when we would give her baby formula but now she is taking puppy formula and she can't poop?

Her stomach has probably got used to the baby formula, and it will take awhile for its stomach to adjust to the new food

Puppies that age often need mommy to help them poo. Mom will do this by licking the puppies bum, stimulating a bowel movement.

You could try using a warm moist cloth and rubbing the puppies bum, that may work.

If it doesn't, please take the puppy to the vet. This can quickly become life threatening at this young age.

Can I ask why the puppy isn't with it's mother?

Alty's advice is spot on (had to spread the rep)

The constipation would most likely be caused by the change in formula.
If the pup doesn't poop after you have tried the suggestion above then get it to the vet, easiest fix is for vet to give an enema (vet only!) and after that slightly increase the amount of water in the puppy formula to prevent it happening again.