Well, it works for me😉
Do you mean, that after you clock sign in, you get another form, that wants you to choose your user name, and password? If yes - it's normal that Flarum wants you to choose your user name after sign up, and it's known bug, that it wants you to write password for classic login. It will probably not ask, if you already have account with this email.
Please delete my account on your forum. I hope it wasn't clever idea to get my email address😉