Hi carissa89,
Welcome to Apple Support Communities. I understand that you're seeing a message saying "Calendar invitation response cannot be sent." I also see that you've tried disabling iCloud Calendars and turning them back on, but the issue persists. I can see why you'd reach out for assistance with this; I'd like to help. I'd recommend trying the steps below for any third-party email accounts you may also be using with Calendar on your iPhone:
- Tap Settings > Passwords & Accounts > Third-party mail account (Gmail, Yahoo!, Exchange, etc)
- Disable Calendar for the account and choose to remove the events from your phone
- Re-enable Calendar for the account and test for the prompt you've been seeing
- When you re-enable Calendar for each account, the events you removed will sync back
Let me know what you find.
Best Regards.