After I created a Repository on Github and tried to invite collaborators, I am shown the message 'User could not be added' .This is the screenshot of the error message I get when trying to invite or add collaborators to my Repository.
I tried changing the Repository from Private to Public, however it is not working. I also tried with a different browser, I get the same error.
I also faced this issue and after connecting with GitHub support they replied:
Restrictions were placed on this account because you have registered multiple free user accounts. Our Terms of Service state that an individual may not maintain more than one:
https://docs.github.com/github/site-policy/github-terms-of-service#3-account-requirements
We'll need to continue to leave these account restrictions in place.
So check if you use multiple free account or connect with their support to get the exact restriction applied on your account to solve this problem.