How can I get a free virus scan for my iPhone?

For all intents & purposes, iPhones can't get viruses.

You don't have any viruses on your phone, it is a scam message designed to get you to purchase unnecessary software or phish for personal information.

Read more about it here -

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

Beware of so-called "virus scanners".

A virus is a type of malware that spreads itself to other pieces of code and scripts, so they can spread more copies of the virus. The name is by analogy to biological viruses such as the common cold virus, which invade cells in the body and turn them into virus factories.

iPhones are very locked down. My understanding is that most apps on a non-jailbroken iPhone cannot modify the files containing other apps. A "virus" cannot spread, and an "anti-virus" app could not remove viruses from other apps, even if there were any.