My little Gabby is a almost 18 weeks old and I am trying to teach her to walk on a leash! She has a harness and she does not mind wearing that but the second I put the leash on it she freezes and lays down and will NOT move

I have tried to get her use to it in the house and outside and she will not move either place. What can I do? I tell her good girl and get excited when she does take a few steps but then she just freezes again. I have an older dog the walks great on a leash and always has so I do not know what to do

Thanks!!!