Message regarding virus installation

It's a scam, here's how to fix it

Delete spam calendars and events on iPhone

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

Ignore it, but Remove Calendar Spam

In Calendar app, tap on Calendars and uncheck the ones you don’t recognise .

Then  Settings > Calendar > Accounts > Subscribed Calendars, look for the problem calendar and delete it.

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