The user cannot be added to the repository

I’m trying to add my friend to my personal storage. However, Github writes: “the user cannot be added.” How can I fix this?

Please note I’ve edited your message to exclude information about another GitHub user.

Typically, you’ll see this message because of a setting on your friend’s account. If they don’t know why you’re seeing the message, please ask them to contact support directly.

https://support.github.com/contact