Every domain needs to be added to the hosting control panel for it to work. A domain name is added to a control panel, which makes services such as email and file housing possible. If the domain is not added, there are no files to be read or emails to function.
For more information on how to add a domain to the hosting control panel, please refer to the following articles:
What is the Difference Between Parked, Addon, and Subdomains?
About Addon Domains
About Subdomains