When using WordPress Multisites with Subdomains, you need to ensure the subdomain is added to cPanel/DirectAdmin to be used.
cPanel
You can do this on cPanel via:
cPanel -> Domains -> Create a new domain
Here you will enter the name for the subdomain and also the document root; this should share the same Document root as the WordPress Multisite:

This will usually be the public_html directory, but if your WordPress Multisite is based on an addon domain will be located on a different path.
If you've already created the subdomain and need to update the document root from the Domains screen, instead click manage next to the subdomain:
From here, set a new document root and hit update:
DirectAdmin
On DirectAdmin the process is similar, add a subdomain via:
DirectAdmin (User Level) -> Subdomain Management -> Add SubdomainOn DirectAdmin, we need to set the Document Root after creating the subdomain, by default DirectAdmin will set this to be:
/domains/yourdomain/public_html/subdomain
Then set the public_html and private_html to custom and to use the same Document root as the WordPress Multisite:

Hit save and wait a few minutes as DirectAdmin build the new document root configuration; you should now be able to visit and use the new document root.
On DirectAdmin, we need to set the Document Root after creating the subdomain, by default DirectAdmin will set this to be: