When trying to import data in a csv file using a unique custom ID field ("VPP ID" on the demo site), no records are imported and all are returned with the following reason in the import_errors file:
Invalid value for field(s) : field ID;
This was recreated on the demo site. VPP ID is an integer field. It does appear in the dropdown menu in the field matching menu as "VPP ID (match to contact)" Other fields matched are total amount, receive date, contribution type.
Original csv shows VPP ID cells to be integer format. Those cells are general format in the import_errors file.
Import test file and error file attached.