"Please check that the File/Reference and the Tax ID of the taxpayer correct"
This error usually occurs when you are trying to respond to a request from the general option "Answer requests or present documentation related to a notification received from the AEAT " inappropriately or when you are entering some incorrect data.
The error appears after clicking "OK" to try to advance from "Step 1: Generation of the request" to "Step 2: Signing and sending the application."
Check the received notification to see what literal options are offered in the notification and whether or not a specific route is specified to answer it.
Also remember that from the "Electronic registration" option you can access the different options contemplated for answering requirements.