Photoshop Elements 2018 & 2020

I have original disks for both of these, and have installed them on two of my machines. One machine is down, so I'm not sure which version is on this machine. I seldom use the program, but it's important when I do. When I tried to open the version on this machine, I got messages about upgrading my operating system, and to update to Elements 2021. As far as I knew up to now, neither of my Elements programs required a subscription. I need to use Elements right now. What is going on here?