Hi Claudia, 

Welcome to Box Community and glad to help! 

This error appears when you're inviting a user with inactive Box account. Please note that for collaboration to work, you should invite a user with active Box account.

Thanks for posting!  

Hello, I have the same question, the user is external, not a managed user. 

The same, user is external and can't invite a deactivated user.

how can I re-activate this external user? Does the user need to take care of this?