Hello kanghsun,
Thanks for that info and for choosing the Apple Support Communities. If I understand correctly, you are having an issue with an error in the Calendar app on your iPhone about a response to an invitation. To help with this, please go to Settings > Accounts & Passwords, temporarily turn off just calendar syncing to any accounts that are currently active, restart your iPhone and turns calendars back on in Settings > Accounts & Passwords and open the Calendar app and test your issue again.
Restart your iPhone
Set up multiple calendars on iPhone
Cheers!