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.
The obligation holds between Belgian VAT-registered businesses. Checked does not know whether this customer is VAT-registered. If it is and is not ready yet, you may send the invoice another way, for example as a PDF. To send it electronically you then need its agreement on a secure channel. The invoice remains valid, and it is then 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.