Best answer by Jonathan
The reason you're unable to rename your account to https://soundcloud.com/lameboi is because this account URL already exists. (as you can see when going to that URL)
I'm afraid the only solution would be for you to try and get in touch with the account holding that URL and let them know why it's important for you to claim that URL, hopefully they would understand and free up the URL for you.
Hope you can get it sorted with them!