I'm not an Apple user, and I will assume you are using the Apple Mail app.
First thing to try would be using a generated password when asked, instead of your usual Sky password.
https://www.sky.com/sky-yahoo-mail/manage-app
However if that does work it means your account was probably not installed correctly.
Second thing (and you might want to try this even if the first worked) is to remove your Sky account from the email app and install it again, and if it asks say the account type is Yahoo.
Third thing is to try using a different app, I would suggest either Outlook or Yahoo Mail, once again making sure they know it is a Yahoo account.
Do let us know how you get on.
