There is only one way to invoice a sale which was performed for a non-guest directly in Point of Sale. In order to be able to do it, you need to set up as default Take Payment & Invoice window in Property Settings / General / Reservation Settings:

Once this feature is active, you can ttake payment for a non-guest directly from Point of Sale:

Once the payment is performed the system will automatically create an invoice. You will be able to see it and open in two places:

1) Accounts & Transactions

2) All Issued Invoices