When the Amazon API status is red, ProductReview shows the exact error Amazon returned. Here are the common ones.
InvalidSignature / SignatureDoesNotMatch
Your Secret Key is wrong or has a stray space. Re-copy it in Amazon (regenerate if needed) and paste it again with no leading/trailing spaces.
AccessDenied / not yet eligible
Your Associates account doesn’t have PA-API access yet. Amazon requires a small number of qualifying sales before granting API access. Keep the account active and re-check.
InvalidAssociate / InvalidPartnerTag
The Associate Tag doesn’t match the selected marketplace, or isn’t approved. Confirm the tag in Associates → Account Settings and that the marketplace matches.
TooManyRequests / RequestThrottled
You’re calling faster than your quota. See Fix “RequestThrottled”.
ItemNotAccessible / no data
Some ASINs aren’t returned by the API (marketplace-restricted, third-party only, or delisted). Try a different ASIN or marketplace.