cPanel accounts must 1st be created before they can be used. This is done from the the WHM interface.
In the WHM interface, go to the Create a New Account section (WHM >> Home >> Account Functions >> Create a New Account).
From here the main settings that need to be inputted are the domain, username, password and email. Once completed, make sure the account settings are correct then click the Create button to make the account
More information can be found at the documentation below: