Why is my phone saying I have a virus when I go to my calendar

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

Ignore it.

Then remove it

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

Don't say yes when websites offer to send you "Notifications" to avoid it again.