You may experience a situation where your MWS keys become invalid and as a result can't run scans. The message reads something like:
Your keys are invalid. Generate your keys again and if the problem persists contact Amazon seller support.MWS Error Message: Access to Products.GetMatchingProductForId is denied
If you do come across this message, which appears in a pink bar to the top of your screen in your account; it means you will need to revoke your current set of keys and generate a new set. See steps below to do this:
1. Revoke MWS keys by going to https://sellercentral.amazon.co.uk/gp/account-manager/home.html and scroll down to FBA Wizard, then click “Revoke Access” (see screenshot below)
2. Go to the API page here: https://app.fbawizard.com/user/api, and go through process of creating the keys again. Ensure to follow the steps on the API page exactly as they are for successful integration.
Note: Keep in mind that Amazon can sometimes take up to 24 hours to grant access to MWS, so if saving the new set of keys fails, wait a while and try again.