Since the merge of user roles management between the appstoreconnect and apple developer accounts, i haven't been able to give access to my team on appstoreconnect to the certificats and identifiers.
It's always disabled and I can't check it.
Simple answer: You can't invite people to resources over at https://developer.apple.com/account with a personal account. Therefore the checkbox is greyed out and not clickable.
Simply go over to https://developer.apple.com/account to check your enrollment. There you can find your "Entity Type" (> Membership)