My Reminders app isn’t syncing / working on my iMac but it does on my iPhone and iPad

If you accepted iOS 13's offer to upgrade Reminders, you won't see them on your Mac until you upgrade it to Catalina.

If you need to see them on the Mac before then, you can access them via:

https://www.icloud.com/reminders

Are your iPhone and iPad running iOS 13? Is your iMac running Catalina?

Get ready for the new Reminders app - Apple Support