Hi, I'm Diane and I'm happy to help you today.
Are you trying to add a new user to the MSN subscription (separate mailbox), not another alias to your own account, correct?
Does the address you are trying to add exist already?
Try logging in using a private browser window to do. If it works, you need to clear the browser cache and cookies. To open a private browser window, right-click on the browser icon on the taskbar and choose the option for a new private or incognito window.
If you are trying to add a new alias to your account, do it from
https://account.live.com/names/manage
As an FYI, if you have 6 or fewer users, a Microsoft 365 Family subscription might be a better deal and it includes the office software.
Let me know how it goes, I'm here to help you further if needed.
-- Diane