You have to be logged into Facebook to access their debug tool: https://developers.facebook.com/tools/debug/
NOTE: The Facebook Thumbnail Fixer plugin has recently been abandoned by the developer so it will soon be no longer recommended by WordPress. Please use the following plugin instead: https://my.wealthyaffiliate.com/training/facebook-sharing-no...
It appears that Thumb Fixer plugin has replaced the Facebook Thumbnail Fixer plugin,
and if you click on the developer's name, it takes you to the Open Graphite developer's web page where they suggest using their plugin, in which their premium version is where you gain all of the necessary options that Thumb Fixer provided.
If you are not using the All In One SEO plugin it would be difficult to use their Feature Manager plulgin shown here: https://my.wealthyaffiliate.com/training/facebook-sharing-no... In this case, your only option is to find a WordPress theme that includes this option, such as the Thrive Themes do.