Photoshop Elements Organiser in MAC High Sierra

Please see:

Elements Organizer 2018 crashes on launch or while importing a catalog

General rule:

Any application like Elements is based on code (instructions, procedures, functions...) which use other code provided by the OS, Mac or Windows. For instance, everything about displaying the interface and fonts is managed by the OS code.

If an Elements instruction 'calls' a given Mac or Win procedure and that procedure is changed or missing in an update of Mac or Win, that won't work. Simple.

Bottom line: don't expect Elements 2018 to be fully compatible with an OS version using new procedures unknown to Elements programmers by the time 2018 was issued.

For the OS suppliers, keeping the compatibility of new versions with any older version is a big problem. To introduce new features, they simply have to remove older features.

How do Microsoft and Apple deal with that 'backward compatibility'?

As a Windows user, I still have old versions of Elements installed and running; PSE 6, 10, 12, 14, 15, 2018. For a number of users, that means some adjustments, but that can work. That means that backward compatibility was fairly well kept... until Windows 10 had to do major changes which required many updates.

On the Mac side, I'll leave Mac users comment, but I can say that 'pro' Mac users I do know never automatically install the latest release.