If a website tells you your Mac is infected with a virus, I would recommend ignoring it. It is highly unlikely that there is something infecting your Mac. Contact Apple Support directly if you are concerned. Do not call or use any link found with the "virus alert."
Apple Support
–or–
1.800.APL-CARE
It is worth noting that there are virtually no Mac viruses, trojans, etc. thanks in part to outstanding macOS security. If you were to run into one of these malicious items, just about anything you ever install on a Mac will require your permission. If you’re not sure what you’re installing, stop immediately and consult with Apple Support.
Trust your Mac’s security and trust your instincts.