Django: TypeError: 'username' is an invalid keyword argument for this function

You have no username field in your model and it's redundant because you have that field already in the User model:

user=User.objects.create_user(username=form.cleaned_data['username'],email= form.cleaned_data['email'],password=form.cleaned_data['password'])
user.save()

employee=Employee(user=user, email=form.cleaned_data['email'], password=form.cleaned_data['password'])
(...)