In order for a Product Identifier to be valid, it needs to have a calculated check digit. A check digit is the last number in the Product Identifier that is calculated using the check digit formula.
While this can be calculated by hand, the easiest way to find the check digit is to use a check digit calculator. Enter the the Product Identifier, for example UPC, without the last digit. The calculator will use the formula to find the correct check digit for your Product Identifier.
Bulk Upload files validate the check digit for Product Identifiers, so you will need to change the last digit of your Product ID in order to upload or update products using the bulk tool.