If you have a site registered and working with a hosting site for foobar.com but want to use a subdomain like sales.foobar.com with Wix it’s pretty easy to setup as long as you don’t only follow their help page and read the specifics. The Wix help page gives you a really good clue in this scenario (which I’m surprised they bother to support) but it’s not complete.
Many hosting companies use cPanel or another tool with a DNS zone editor. The instructions have you point the domain (sales.foobar.com) by creating an “A” record to a specific IP address. You also need to add a “CNAME” record for the www prefix (which is easy to overlook in the instructions). In my example would be setting www.sales.foobar.com to their server. Unfortunately the instructions leave out another domain name you need a CNAME for which is m.sales.foobar.com. Their help guide misses this one so to see details of each of the CNAME records and the A record is to “My Account / My Domains” and click on the advanced tab. Open up each of the collapsed items for what I have mentioned above. The also suggest you change your name server, but of course if you main domain is hosted elsewhere you won’t do that.
So in summary – log in to Wix, go to the “advanced” tab of your domain and set you hosting company DNS zone file for the A record IP, and the www. and m. CNAME records and you’ll be set. The whole thing should take 10 minutes. It seems the “Check Connection” tool is trying to validate the DNS entry they don’t control (foobar.com) or my changes haven’t propagated to their servers yet.