Actually when I do a server header status check on that subdomain I get a request failed error. It’s not returning 404 but a generic server error.

This usually means something is wrong with your server or domain configuration.

So based on this, below are few suggestions:

• Review your .htaccess to see if there are some missing lines or wrong syntax. WordPress does provide a standard .htaccess syntax during multisite installation. Make sure you copy and paste this as exactly what is suggested.
• Double check if the site exist on multisite network admin. When the site, exist its listed on Sites -> All sites.
• Review the site settings, check for wrong site URL ,etc. You can view this in the Edit site settings in network admin.
• This maybe rare, but some hosts don’t allow multisite configuration. If all things fail, try contacting your host regarding this.
• Review if you have correctly created the wildcard subdomain. There should be an A record for *.yourdomain.com created and pointed to the server IP address (it could coincide with the IP address of your main domain. Tutorial: https://www.namecheap.com/support/knowledgebase/article.aspx/9191/29/how-to-create-a-wildcard-subdomain-in-cpanel