The user is trying to create a client profile, but gets an error that the client already exists. However, there is no client in the list view that has this same ID.
This can happen if a client with this name already exists, but is inactive. Core only displays clients with an Active status in the list view and drop-downs. So, you should choose a different name for your client. You might also want to check the existing client to see if it might actually be the one you are trying to create. You can view inactive records in the list view using the filters. You can delete the client from Core and create a new one or change its status back to Active.