Have you correctly connected Correos but you're facing some issue during label creation?
Here's how you can fix them case by case:
El envío no se ha registrado. Es necesaria una descripción de la mercancía para este tipo de envío
You can easily get through this error by adding the Code or the Description of the product in the Description Field taking into consideration what directly provided by Correos / your Correos Sales person or the codes you can find inside this Correos website page siguiendo la ruta Annexes > Goods table.
El envío no se ha registrado. El nombre del remitente o la empresa remitente es obligatorio
In this case please make sure to fill EMPRESA (Company) field.
Problems creating SAAJ object model
Check the Recipient name field for any special characters or extra spaces, then proceed to delete them and retry.
Error: El envío no se ha registrado. La oficina de destino no es válida
In this case, you can proceed to create a new shipment for the order again