The unknown user error can appear if the patron's barcode that appears in the ComCat request is not the same as the patron's barcode in your ILS. This can happen if the patron's card has been recently updated in your local system, and they placed the ComCat request with their old number.
The error may look like:
-- NCIP error: NCIP AcceptItem: Cannot receive item. (9004; Unknown User; )
-- NCIP AcceptItem: Cannot receive item. (Unknown User; User with barcode XXXXXXXXXXXX unknown; UserIdentifierValue; XXXXXXXXXXXX;) or NCIP AcceptItem: Cannot receive item. (Unknown User; AcceptItem; )
If your library is part of one of the following networks: CW MARS, MBLN, MVLC, NOBLE, and OCLN: You have the option to update the request yourself. Please find the instructions below.
All other libraries: Please submit a ticket to the Helpdesk with the patron's new library card number so the request can be updated in ComCat with the patron's new number.
CW MARS, MBLN, MVLC, NOBLE, and OCLN:
Networks with the patron user lookup enhancement have the ability to update the request with the patron's new number in ComCat. From the staff dashboard, click on "Blank ILL request" in the top menu. Enter the patron's new number and select "Barcode." Click the search button.

If the correct patron appears in the results, select the patron, as if you are going to place a new request for them.

This will take you to a blank ILL request form.

Going through the user lookup process is necessary to update the patron number on the request. Click "Cancel" on the blank ILL request form, return to your staff dashboard, and re-open the original request. The patron number on the request should be updated. Try to "Receive" the request again.
If this is not successful, submit a ticket to the Helpdesk.