Hi, I'mCustomer Welcome to JustAnswer. I'm reviewing your question now, and will post back with your reply ASAP.
I'm sorry about the issue, Backup your data then go to Settings > iCloud, uncheck Calendars & Reminders, choose Delete from Mac at the prompt, restart your Mac, then go back and check Calendars & Reminders again
let me know the results.
Has everything worked out, do you still need help please?
let me know if you still need help