Sharing List Microsoft To Do

Hey Guys,

I am using a private for Microsoft Office To Do. I wanted to share a list in MS To Do with a friend, but he can't get into it because of a message saying 'Can't join'. I already asked the MS support for help and tried to figure out if the sharing link is closed already. But all of them didn't work out. So do you know the reason?

Thx a lot.