“My invoice has been stopped and I get a message saying that the recipient is not in my customer directory, but I know that the recipient exists.”

The recipient might be in your customer directory, but there might be values in the invoice that doesn’t match your recipient.
Make sure that the customer number you have added to the invoice is the same customer number that’s in your customer directory. If you use matching by address, make sure that the address you’ve added in the invoice address is listed in your customer directory.
If you use organization number, VAT registration number or EAN on the invoice, make sure that the value also is added in your customer directory.