Maybe you have a key (or too many) pressed that is not allowing you to type? I forget what it is, but there is something that makes that beep noise and you won't be able to type. OR, press and hold delete and backspace for a while, because maybe you have tons of color codes still in there (which would be invisible), and that takes up a lot of space for the max characters for the username. Make sure everything is deleted, and start over.

If you still cannot do anything, go into your cfg.txt file and delete what is on this line (in red):

and just type something simple there like "name". Then go into LFS and see if you can delete that and start over. Good luck.