Many Safari users found that after upgrading to macOS Sierra, suddenly websites tell you that they require Adobe Flash Player, Silverlight, or Java. You download it, install it, and it still doesn’t work.
In reality, the plugins install just fine, but Safari has become far more aggressive about allowing them to load. It should be asking you the first time you visit a website if the requested plug is allowed to run. But on many computers I’ve seen the plugins completely disabled.
To fix the problem open Safari and click the Safari menu > Preferences > Security tab > Plug-In Settings. On the left column make sure the checkbox is marked for the offending plugin. Then click the name of the plugin to pull up the settings on the right side. Go through the list and turn on the plugin for every website that should be allowed to use the plugin without prompting you. Or choose “ask” if it should ask your permission each time. Below that is an option for “When visiting other websites”. Choose “On” to automatically allow the plugin to run on new websites, “ask” if new websites must first get your permission, or “Off” to automatically decline new websites.
You may need to visit this panel from time to time if new sites are giving you a hassle.
Just another chapter in Apple’s war on Adobe. I’ll be glad when Flash is finally gone.