Error 6 (whatever that means) unable to open old Photoshop Elements

I have Photoshop Elements 8 on a Mac mini. I bought it when I had another computer. I have used it on this mini, but recently had all files removed and a new OS installed. I re-installed the Elements program. Today I tried to use it and got a message saying it could not be used, something about license and gave a helpful "Error 6" message. I chatted with an Adobe help agent who gave me a link to a download page for Elements, but not instructions. I downloaded a copy of Elements 18 (I think). When I clicked on the Elements Icon (I only saw the old one), I got the same message. Now I wonder what I need to do to get the new software to mount.