Cannot remove an adminstrator

I purchased a ghost site from a marketplace. He has been the administrator and the owner. He transferred the ownership to me, but he is still the administrator. When I attempted to change him as the contributor and the author, it gives me the an error message. How am I supposed to change myself as the adminstrator?

*this is the error message “Validation error, cannot edit user. Only one role per user is supported at the moment.”

How about suspending the unwanted user and making a new one, if the role change isn’t working?

Thanks for your response. If I suspend them, the articles won’t be affected, right?

Correct. And I need 20 characters to make a post, so still correct. :)