I received a message from a site entitled macupdate.com, indicating I had a virus, and needed to do a complete system scan. I didn't open the message, but could it have been legitimate?

No. It is a total scam. You can report the phishing attempt to Apple if you wish:

Identifying fraudulent "phishing" email - Apple Support

Phishing & Other Suspicious Emails

Good job on recognizing that it was suspicious. There seems to be an uptick in this particular phishing scam, and I can't tell you how many people have not only opened the email, but have called the number and allowed the low-life's access to their computers.... 😟

Best of luck,

GB

The rule of thumb with MacUpdate is not to have your mouse pointer inside your browser window while that site loads. If you do, the odds are that you will trigger an unplanned, roll-over, advertisement. Ignore these at all costs. I have been visiting the MacUpdate site for years, and they never have initiated any contact with me. I suspect that you rolled over one of their annoying third-party advertisements, and it popped an intimidation/buy message on you.

I visit that site daily to keep track of trending software versions. Any that I download, are directly from the original vendor site, or if available, from the OS X App Store.

You are most welcome! 🙂

Best of luck,

GB