WordPress Multisite Wildcard Subdomains on Plesk

You have forwarded the wildcards to the wrong subdirectory this is why you are seeing the Plesk default page. The wildcard subdomain rule should point to the folder of your wordpress multisite installation.

WordPress reads the URL from the request and then internally routes to the correct site. If it’s not pointed to right place then this can’t happen.

I think this probably means the CNAME is done correctly but you do not have the DNS entry set up.