The full error message is "Recipient of document did not match any recipient in Pagero public customer registry or in your customer directory. Verify that the recipient is added to your customer directory. Also verify that customer numbers or any other identities such as organization number, VAT number, etc. matches." and can happen for a few different reasons.
Firstly, the customer might not exist in Pageros public directory at all. If that is the case, you can follow the instructions here to suggest the customer be added to the directory.
Second, you might have a closed customer directory, but have not added the customer to your private customer directory in Pagero Online. To do so you can follow the instructions here.
Lastly, if you have an open customer directory, or you have verified that the customer exists in your private customer directory but still get this error it means Pagero Online was unable to match the values given in your invoice to the values found in your customer directory. Make sure that the customer number given in the invoice is the same as the customer number added to your customer directory in Pagero Online. You should also verify that the different identifiers you include in your invoice, such as organisation number, VAT registration number, or GLN/EAN, match the information for the customer in your customer directory in Pagero Online.