If you receive the message that "your company is already registered", it means that your company is already connected to Peppol in another Doccle account.
The fact is that only one Doccle account can be linked to a company for Peppol.
What can I do ?
Within your company, verify which of your partners has already activated the Peppol connection in his or her Doccle account. You can then ask them to Share the connection with your Doccle account.
Thanks to Sharing, several people within the company can view the Peppol connection and documents in their own Doccle account.