In order for a client to be coached, first, the client profile needs to be active. If the clients' profile is active you will be able to assign a coach to the client.
Read here how you can activate a clients profile.
To assign yourself or another coach to a client, go to the Clients and Staff module > Client overview and then to the Clients' profile.
Select the 'Coaching' tab, scroll to 'Coaches' and here you can assign a coach to the client.
Tags: Assign, coaching, training, trainer, designate, linking