How to enable Flash Player plugin for Safari in macOS Catalina?
I'd start by uninstalling Flash completely - very few web sites use it any more. If you don't have it you will see a clear indication if it's required.

Get the uninstaller from Adobe - Uninstall Flash Player | Mac OS

Warning: Do not fall for any pop up telling you Flash is needed or out of date - they're scams, malware. If Flash is legitimately needed for a video, the placeholder for the video will say that Flash is required; usually quite simply, "plugin missing" or similar, possibly with a 'jigsaw puzzle piece' icon.
It will not offer to go get it for you.

If you ascertain Flash is actually required, make sure to fetch it directly from Adobe, don't click through any links you cannot see clearly.
https://get.adobe.com/flashplayer/

Then to make sure it's enabled in Safari, go to Preferences > Websites. Scroll down the list on the left until you find Adobe Flash; select, then to the right, decide whether you want behaviour on, off or ask by default &amp; for any specific sites. 

Full instructions at Adobe: Enable for Safari

Note that Flash is very nearly end-of-life &amp; most browsers are going to start actively blocking it over the next year. If you're not certain whether you currently still need it, work without it.