You attempted to add a transaction to the register but QuickBooks does not recognize the payee.
Create an alias for the payee to an existing person in QuickBooks (such as a customer or vendor).
Quick Add a name for the payee only, without entering detailed information now.
The Quick Add button appears on the message QuickBooks displays when you
enter a name, item, class, account, customer type, vendor type, payment method,
payment terms, or shipping method that QuickBooks has no record of.
Clicking Quick Add creates a new list entry based on what you enter into the
form you are working with. For example, if you enter a new customer name on an
invoice, clicking Quick Add adds the new customer to the Customers & Jobs list and
adds to the customer's record all information you enter about the customer
on the invoice form (billing address, payment terms, taxable status, etc.).
Set Up detailed information for the payee now.
Clicking Set Up takes you to another window where you can enter information
not covered by the current form. In the case of a new customer, for example,
you could click Set Up to enter the customer's phone number, fax number,
and credit limit. If you choose Quick Add instead, you can always add more
information to the customer's record at a later time.