multisite (MU) Subdomain- only one wordpress install

Your problem is not a WordPress problem, but a server setup problem.

WordPress is perfectly capable of handling the requests to the subdomains, but those requests never reach WordPress. Your server is not configured to map those subdomains on to your WordPress install. That’s why you get the placeholder, as that’s what your server is setup to present when it’s asked to serve a domain it doesn’t know how to handle

To fix this will require adjustments to the Nginx configs, or Apache vhosts, so you will need to contact your hosting provider, or make the relevant changes yourself