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This is usually caused by outdated flash player. We can uninstall old flash player and install a new version.
Click this link and this will download the flash player uninstaller file --> https://fpdownload.macromedia.com/get/flashplayer/current/support/uninstall_flash_player_osx.dmg
After downloading,click on the Finder icon on dock, then click on Go menu and select Downloads.This will bring Downloads folder. Now you see a file named "uninstall_flash_player_osx.dmg"there. Double click on it to open. Then open flash player uninstaller file. Let it uninstall
Then download latestflash player from this link -->
After downloading, click on the Finder icon on dock, then click on Go menu and select Downloads. This will bring Downloads folder. Now you see a file named "install_flash_player_osx.dmg"there. Double click on it to open. Then open flash player installer file and install it.
Another option is,you can download another browser like Google Chrome and use it to browse internet. It has the plugin inbuilt and it will work. You can download Google Chrome from here -> https://www.google.com/chrome/browser/desktop/
If you have any difficulty on following the steps above, please let me know and I will walk you through it.