Being listed in the Peppol Directory is voluntary, so the company can still receive invoices via Peppol.
Invoicing without a Peppol listing
Try sending from your invoicing software. If that fails, ask the customer whether they already receive via Peppol.
If the customer is not ready yet, you may send the invoice another way, for example as a PDF; if you send it electronically, you need their agreement on a secure channel. The invoice remains valid; it is the customer, not you, who risks a penalty. FPS Finance
Follow this company and you will hear the next morning if it appears in the Peppol register.