If you take the URL out of the $urlfetch and just go to it in your browser replacing all the variables like user and querystring with your username and a real levelcode. What kind of response do you get from the server?
Also I just have to ask, are you trying to use this with Super Mario Maker 1 or 2?
So, that would be why. https://warp.world/multiqueue is the solution for Super Mario Maker 2, it’s a generic queue system that currently only works with our WarpBot. You need to setup things there, the main site links you to MultiQueue explaining that the old site is still only just for Super Mario Maker 1.