There are a few possible causes for this issue:
1) The Seller ID is incorrect. Your Seller ID should be a 13 digit combination of letters and numbers, typically starting with the letter 'A'. Your Seller ID is NOT the name of your Amazon store or eBay username. You should be able to find it where you found the rest of your Amazon credentials.
2) There is a typo in your MWS credentials. Please double check the MWS settings on your settings page, and make sure they are entered correctly. Also, double check that the credentials are indeed the proper credentials, as Amazon MWS is not the same as your general Amazon seller account's credentials
See this article to double check your process: https://joelister.zendesk.com/hc/en-us/articles/115004183354-How-do-I-find-my-Seller-ID-Marketplace-ID-MWS-Access-Token-with-screenshots-
3) Our system cannot be authorized despite proper credentials. This happens very rarely, but on occasion, we have found that users cannot sync their Amazon data despite proper credentials. Contact Amazon support and ask that your MWS "secret key" be reset. Once the MWS secret key is reset, re-input the field with the new key. After saving MWS settings, press "Update Inventory" button to make sure your inventory is pulled.
If you continue to receive this error, file a support ticket for further investigation.