Callander warning that I can’t delete.

If running iOS 13 or earlier versions, check: Settings - Passwords & Accounts - Accounts - Any rogue entries here? If so, delete the rogue account. 

If running iOS 14, check: Settings - Calendar - Accounts - Any rogue entries here? If so, delete the rogue account. 

The Apple support article is below. 

https://support.apple.com/en-us/HT211076

This is called Phishing

You've clicked on something online that granted access to add an account to your device.

FOLLOW THE FOLLOW STEPS TO FIX!!!

Delete spam calendars and events on iPhone

If you get unwanted calendar invitations or event notifications, you might have accidentally subscribed to a spam calendar. To remove the events, delete the calendar subscription.

How to remove calendar spam on your iPhone

1. Open the Calendar app.
2. At the bottom of the screen, tap Calendars.
3. Look for a calendar that you don't recognize. Tap the More Info button  next to that calendar, then scroll down and tap Delete Calendar.

If this doesn't fix the issue, delete the calendar subscription in Settings:

1. Open the Settings app.
2. Tap Calendar > Accounts. Or if you use iOS 13, tap Passwords & Accounts > Accounts instead.
3. Tap Subscribed Calendars.
4. Look for a calendar that you don't recognize. Tap it, then tap Delete Account.

https://support.apple.com/en-us/HT211076