Once a username or password is registered onto the system, it is then marked as taken and cannot be used again.
Two things could have happened here. Firstly, you may simply be choosing a username and password that has already been taken by another member
You have tried to sign up and that sign up failed. When you try again, it says your username and password is taken.
Can you please just check with your bank and make sure that a transaction was not processed and then try again. You will need to choose a significantly different username and password to successfully join.