Hi kgustin,
Thanks for posting in Apple Support Communities.
We understand that you aren't able to share to Reminders on your Mac and that it's also not shown in the Share card to be added. Be sure that the built-in Reminders app is installed. If you're using a third-party installed app, this may require additional settings to be enabled.
If you choose not to upgrade your Mac, make sure that you're running the latest version of Catalina. Restart your Mac to safe mode and test for the ability to share. If this continues, create a new user and check whether you're able to share from the new account.
"How to use safe mode
Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:
Apple silicon
- Shut down your Mac.
- Turn on your Mac and continue to press and hold the power button until you see the startup options window.
- Select your startup disk, then press and hold the Shift key while clicking “Continue in Safe Mode.”
- Log in to your Mac. You might be asked to log in again."
Find these steps in How to use safe mode on your Mac.
Regards.