Here's an overview of this error and how to fix it:
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Error Message: Input data for tag <Item.ProductListingDetails> is invalid or missing. Please check API documentation.
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Response data: <LongMessage>UPC has an invalid value of {#######}. Enter a valid value and try again.</LongMessage>
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Overview: In this case, eBay's custom label field for this SKU is either blank or invalid. In Zentail, this is the UPC field.
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Solution: To fix this, simply enter a valid UPC for your product. If the product does not have a UPC, enter "does not apply" and then save your edit.