how can i check for viruses

Your phone does not have a virus.

The "message" was a common scam that is trying to trick you into buying useless software or worse.....getting you to reveal personal information like your Apple ID and Password.

Never click or tap on a link in a message like this.

Close the message, delete if possible and move on.

Do the same for other similar scams.

There are NO known viruses for iPhone. All such messages you see are SCAMS designed to scare you into buying software you don't need and simply won't work on an iPhone. Simply ignore these scams and NEVER interact with them or EVER give them any information.

Read this --> Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

All such messages are scams presented by criminals. Close the offending page and avoid sites that are involved in such bogus advertising tactics.

Phony "tech support" / "ransomware" popup… - Apple Community