Eating faeces (also known as coprophagia) is a common behaviour for young dogs. The majority of the time it is a behaviour issue and not a dietary issue.
We don't know exactly why puppies eat poo, although there are many possible reasons. Young dogs may do it because they are inquisitive and like to 'try new things'. Some dogs may also do it because they are natural scavengers and will literally put anything and everything into their mouth.
Eating poo may also be a submissive/dominance behaviour, where the puppy is 'cleaning up' to remove its smell and other unusual smells from the area.
Rest assured if your puppy is on a quality dog food they are unlikely to be missing any essential nutrients in their diet; adding supplements is not recommended.
To stop your puppy eating poo, as soon as you see her go to the toilet try and pick it up immediately to break the behaviour cycle. Also, let her know it is unacceptable by saying 'NO' in a stern voice every time she tries.
Often puppies that eat their own poo are also into eating other rubbish. Keep general rubbish out of her reach and give her something engaging to keep her entertained, like a treat balls or chew kongs.
Usually most puppies will grow out of the behaviour as they get a little older and less inquisitive, but trying to stop the cycle can speed up the process. Unfortunately there is no magic trick that will stop the behaviour, it will take some time and persistence.