Adobe has decided discontinue support for Flash as of January 12th, 2021. As a result, the latest version of Flash (220.127.116.111 or newer) contains code that will prevent Flash from operating after that date.
JPOS requires a functional Flash install to operate. If you have the latest version installed, you will need to uninstall Flash and revert to an older build (18.104.22.1683 is recommended) in order to access JPOS after January 12th.
If you use a browser that prevents Flash from running (Firefox, Chrome, etc), you will need to install a Firefox clone such as Palemoon or Basilisk to access JPOS.