I have an older iMac that worked great with PSD CS6,

It might depend on the version of macOS that is on your iMac. Adobe has been supporting the last three versions of macOS. They shifted up one version when Apple released macOS 10.11, so if you look at the Photoshop system requirements for the latest version, Photoshop 2021 requires macOS 10.14, macOS 10.15, or macOS 11. If your iMac was running an older version, the latest version of Photoshop won’t be listed for installation.

The solutions are to upgrade the version of macOS on your iMac if possible, or if you must keep your iMac on the version of macOS that it has now, continue to use an older version of Photoshop.

The Apple system requirements for macOS 10.14 say that it works with iMacs going back to late 2012, so if your iMac is newer than that and you upgrade macOS, it will be able to run the latest version of Photoshop.