Can you ask your wife to open the link in an incognito/private window to see if the issue persist?
On another note, was your wife part of the other family plan? Premium Family members are only be able to switch plans once in 12 months, this includes switching between Premium Family and Duo. They can return to the plan they just left, but will get an error if they try and switch more than once.
Keep us posted.
Help others find this answer and click "Accept as Solution".
If you appreciate my answer, maybe give me a Like.