It’s because you are in the UK
Yahoo! official support need to get their act together. They really are pathetic. Only US users are able to edit nicknames, UK users are not. You would expect their support to know this, and not pass you rubbish information such as uninstall and reinstall etc…
Yahoo! support is full of auto replies, and lenghty delays, whereas unofficial websites and forums like this one are almost instant, and full of facts!
You would need a US account (i.e. you say you’re american when you sign up) to be able to edit nickname.