You are trying to select a Client Contact role for a client that has no Client Users.
Client Users are invited, by you, to become a user in their individual XBert Client Console. You may invite one user or multiple users for each client.
You can select to hide XBerts and tasks from clients.
Note: Make sure you only invite clients to their own Client Console. You don't want to invite them to the CONNECT Portal this is just for your internal team and it gives access to all files.