The opening code is sent directly by the carrier, by SMS or email, when the parcel arrives in the locker. Check your junk folder and the mobile number given when ordering. Some carriers (Mondial Relay, InPost, etc.) also show the code in their own app; for a parcel carried by Colis Privé, however, the Mondial Relay app cannot retrieve it — rely on the SMS and the email.
Without a code, contact the carrier using the details on your tracking page: only they can resend the code. Is the locker full, out of order or not opening? The locker is also managed by the carrier: contact them on the number shown on the delivery card. If the parcel goes back because it was not collected, report it from your order page.