Please go to revseller.com in your Chrome browser, then click logout, then log in. Then check an Amazon product page and see if it works.
In a few instances, we have found that completely closing the Chrome browser and restarting it can be helpful.
Depending on the issue, sometimes clearing your browser cache/cookies can be helpful (link: chrome://settings/clearBrowserData). Please be aware if you choose to do this, it will delete your cache/cookies from other sites as well.
Please go to RevSeller.com, click on "My Account" and then click on "Amazon MWS." That page will tell you if you need to add or update your MWS credentials in order for the extension to work.
Go to chrome://extensions/ Make sure RevSeller is there, and make sure on the right side the checkbox is checked so that RevSeller is enabled.
Go to chrome://extensions/ Find the Revseller extension and click the trash icon to the right to uninstall. Go to RevSeller.com, click "My Account", then click "Download Extension" and follow the instructions there.
Go to chrome://extensions/ and disable all other extensions (uncheck them on the right side) except RevSeller, then try and reload an Amazon page and see if it works then.