Ignore it.
You got lied to.
Have you ever met a web pop-up that was anything other than utter rubbish?
Websites cannot scan your device for malware. The depth of access required for a malware scan would also allow the scammers to just upload everything; which they’d just do and not bother with the pop-up. But since they can’t, they try to hack us, you, and me, and everybody else, not our devices, but us, into doing things against our own best interests.
Here are two links to this and other scams, and how we’re all targeted for scams:
Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support
Scam Pop-ups, Scam Viruses, Scam Receipts… - Apple Community