In Mac OS 10.13.6 High Sierra, when I try to uninstall Flash Player, after opening the Uninstaller and clicking on Uninstall, then I have to enter my computer password. As soon as I do that, I get the little colored rotating ball indicating the app is not responding, and the only choice is to Force Quit.
I suppose the root of the problem could be this: I already uninstalled Flash Player, but I think I did that after upgrading to OS 11 Big Sur last month. But I had problems with Big Sur, so today I downgraded back to High Sierra by restoring from Time Machine, so I've gone back in time to when I hadn't yet uninstalled Flash Player. OR alternatively, I may have uninstalled while still using High Sierra, but when I restored from Time Machine it was not to my last backup (because restoring did not work the first time I tried) but to an earlier backup from October, prior to having uninstalled Flash Player.
I hope I can get some help.
Of course, I had already done exactly what you suggest—including rebooting. This time, however, it worked! The reason, I'm pretty sure, is that I didn't have any browsers open. Normally, the installer will tell you to close the browsers, but in the apparent unresponsiveness of the Installer app, it never got to that message. This time, after again getting the .dmg from the website, I was clever enough to close the browser before opening the uninstaller.
Anyway, thanks Jeremie. Indirectly, your advice worked.