how do I get flash player videos to work on my Imac

Flash videos can no longer be played by any platform, macOS, Windows, Linux. Flash has been purged from the internet. Adobe stopped development months ago. If those videos are truly Flash (which almost certainly they are NOT) then no one can play them, not only you.

If the content uses Flash, contact the website staff; it's no longer possible to use that plugin. Otherwise, it depends on what happens when you try to play it.

(198063)

Do you have any third-party useless-ware like anti-virus or so-called "cleaning" apps installed? If so, remove them with extreme prejudice using the developers' instructions.

What about ad blockers? There is emerging evidence that having more than one ad blocker can cause browser issues.

Any othe Safari extensions installed? If so, disable them one by one as see if and when the problem resolves. If it does, the last extension disabled is the one causing the conflict.

Also check for adware with EtreCheck. It is safe and developed by a valued and long-serving contributor here.

Try using VLC.