If General Journals involve an account of type “Accounts Receivable”, “Accounts Payable” or “Payroll Payable” the Name field in the journal needs to link to a valid customer/supplier/employee.
The journal will create an accounts receivable entry for the name selected. i.e. An invoice.
You can either create a customer called OpenBalance or something and connect the full amount to that or you can have individual journal lines for each customer balance which is a lot cleaner when you start to receive payments.
Here's a HELP file to explain further