Marketplace API credentials are required for Amazon Analyzer to access data from the Amazon API. These credentials are found in Amazon Seller Central and require a professional seller account. AMZ Analyzer pulls all data for its FBA fee calculator, including FBA shipping costs, and all other fees directly from Amazon using these credentials.
Invalid Marketplace API credentials errors can have a limited number of reasons, which are listed below:
1) Credentials are being rejected by Amazon API due to being invalid or inactive credentials,
2) Your computer system time is not current. The API requires a current (live) system date and time. See if your system date/time is set to “automatic” in Windows,
3) Internet connection or Amazon API downtime issue,
4) Various possible Windows glitches, these are resolved with a simple restart.
Pay particular attention to #2 and make sure your system time and date is accurate, the Amazon API requires it. Pay particular attention to AM/PM which often gets overlooked ;).