Check for URL errors and make sure you're specifying an actual web page file name and extension, not just a directory. Most websites are configured to disallow directory browsing, so a 403 Forbidden message when trying to display a folder instead of a specific page, is normal and expected.
If you operate the website in question, and you want to prevent 403 errors in these cases, enable directory browsing in your web server software.
Clear your browser's cache. Issues with a cached version of the page you're viewing could be causing 403 Forbidden issues.
Log in to the website, assuming it's possible and appropriate to do so. The error message could mean that you need additional access before you can view the page.
Typically, a website produces a 401 Unauthorized error when special permission is required, but sometimes a 403 Forbidden is used instead.
Regards,
Will