I installed Monterey on my new MacBook 14 inch M1 Pro (What a lovely machine!) and I wanted to share an email to Reminders - but I can not find Reminders in the share Menu. Under System Settings - Extensions - Share Menu, there is NO Reminders to be seen.
Does reminders appear in any Share sheet – like in Safari or Pages, etc.?
You could try to reset the privacy settings for Reminders with this command in Terminal
tccutil reset Reminders
If you thing more than just Reminders is hosed, then very thoughtfully try to reset everything
tccutil reset all
This last one will result in a lot of clean up and prompts from that lose their permissions to look at files, control the interface, etc., so it is a last-resort type of action.
If your issue is that you do not have Reminders in any share menu, I do not have a suggestion. But if your issue is with Mail, you might try this “workaround”. Highlight the subject of the email (or any text in the email) and right-click and Share to Reminders. A link will be created in reminders back to the email.