Your ASIN creation process might have failed due to the identifier for your product already being used on a different product in the Amazon catalogue. The offending ASIN may be active or inactive.
If the ASIN is active, do not attempt to modify the current ASIN to match your product – rather escalate the violation to Amazon Seller Central to authenticate your license of a GS1 Company Prefix or GS1 US GTIN. Amazon can then go into existing ASINs incorrectly bearing GTINs from your prefix and suppress the GTIN from these listings blocking your ASIN creation.
If the ASIN is inactive without any stock in Amazon warehouses and gets trickier if there are active ASINs with stock that is not using an FNSKU (stored by the GTIN).
Please reach out to GS1 US at email@example.com for assistance in resolving this matter.