Please explain to me what you mean!

What part of "Move the code that checks if a username already exists and adds a user to the database and redirects to come before you print the header" is difficult to understand? Now obviously you'll also have to move the part of the code that assigns query parameters to variables too, but all that you have to do is move two pieces of code. That's it.

It would be much, much easier to help you if you were clear on what you do not understand rather than saying "I can't do that".