cPanel accounts must be created before they can be used. This is done from 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 must be input are the domain, username, password, and email. Once completed, make sure the account settings are correct, then select the Create button to make the account.
More information can be found in the documentation here: Create a New Account.