How would I create a while loop for a log in page /u/Separate_Station1906 Python Education

I am beginner and I am trying to create a log in page. What I’m struggling with is how I can create a loop, where if the user uses their name while creating a username, the code will loop them back to trying again

I am beginner and I am trying to create a log in page. What I’m struggling with is how I can create a loop, where if the user uses their name while creating a username, the code will loop them back to trying again