SAP Message VG520 - & is not assigned to an ISO country key (header, function &, field &)

Diagnosis The specified internal country code has not been allocated an ISOcountry code. The error has arisen in the document header for thespecified partner role and the specified field. Procedure Maintain the ISO country code. INCLUDE 'NA_CUST_MAINTAIN_3' OBJECT DOKU ID TX LANGUAGE E