Shared calendar not working

I have a shared calendar with my husband. We got an au pair and I am trying to add her to the calendar. I saved her non-iCloud email in my contact for her and sent her the invite to the calendar. No invite popped up in the calendar app, but she received an email invite to the calendar. When she selects to accept the invite it says she isn’t invited.

In my invite she has full access to the calendar. We both have iPhones. I am mot sure why she can’t accept the invite.

I even tried making a new calendar to add her to and she still could t accept the invite.

Is it because I am using a non-iCloud email? Is it because her iPhone is from France? Please help.