my iMac's OS is Catalina 10.16.7; what do I have to do to get Adobe Photoshop Elements 9 to work?

You’ll need to purchase a newer version of Photoshop Elements.

You can run the latest version Elements, 2020 as Adobe states here.

https://helpx.adobe.com/photoshop-elements/system-requirements.html

If you’re thinking of purchasing and older version of Elements, here are the OS requirements for older versions,

https://helpx.adobe.com/photoshop-elements/system-requirements/older-versions.html

I hope this helps.

I believe the oldest version of Photoshop Elements that is 64-bit (Catalina-compatible) is v12, and only if updated before installing Catalina.