Is there High Sierra 10.13.6 anti-virus software you could recommend?

Do not install third party antivirus apps. They use system resources while providing no benefit. And they may cause problems.

I've been running Avast Premium Security for a good while and like it. Macworld has an updated review of AV software it considers the best. You might check that out.

Your computer should not need any AV apps. If you are that concerned then please either update Mac OS if your machine still supports updates or please replace the computer with something more up-to-date.