Antivirus

Go to Settings/Passwords & Accounts - Subscribed Calendars, and delete the offender.

And next time don’t accept any free offers from apps or websites. And if you get a prompt asking if you want to install something say “no"

Edit: previous reply answers this.

After you sort that calendar, read this:

Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

There are a number of organizations trying to scam you out of your Apple ID.

Should your device be stolen, pretty much every request received will be an attempt to get your Apple ID. They’ll pretend to be Apple, cops, anybody,...

You can't get a virus on your device.

You can't have a virus.

You probably have calendar spam.

Delete the calendar you accidentally subscribed to.