How do I report a buyer who is asking for refund or he will give 1 star review?

This can result in a review/rating manipulation if you send refund they give 5-star, so for this you can contact Amazon seller support, provide them the evidence that the person is threatening, and hopefully they can sort out this problem of yours.

If the buyer continues to threaten a negative review or engages in abusive or harassing behavior, you can report them to the Amazon’s customer support team. Be sure to provide any evidence you have, such as screenshots of messages or transaction details.

If a customer leaves a bad review and requests a refund, you must open a case or contact Amazon Seller Support to resolve the matter. But, you must be sure to supply Amazon with sufficient proof because customers are Amazon’s top priority. Amazon usually looks out for its consumers, and it will only delete a negative review if it violates its community standards.

If you are a seller on a platform and a buyer is threatening to give you a 1-star review unless you provide them with a refund, you should first try to resolve the issue with the buyer. If you are unable to resolve the issue with the buyer and you believe that the buyer is engaging in fraudulent or malicious behavior, you should report the buyer to the platform’s customer support team.