How to make a new Yahoo.co.uk account?

I already have a yahoo.co.uk account but would like to make a new one for my mum. When I go to the Yahoo page where you can create a new account, I have put in her name and surname. It then wants me to enter the Yahoo details that I want to use - I've put in the name that my mum wants me to create (ie. nevermissadeal@yahoo.co.uk), but then I get a message up saying, 'You can only use letters, numbers, full stops (‘.’) and underscores (‘_’) in your username' - does anybody know why this is happening. Further down the page it goes on to ask for a password and mobile phone number, but because of the previous problem I have not got this far. Any suggestions as to how to get around this? Thank you.