Login to your cPanel account and look at the left hand menu. It will list the account stats, what has been used and how much of each feature you have remaining.
Notice how the below example says 0/0. That means you are trying to add something your account does not allow.
Please go into your WHM and create a new hosting package, or edit an existing one to have the features you want.
Finally, upgrade your cPanel account to the new or updated package.
Example:
Disk Space Usage 0.41/1 MB
Monthly Bandwidth Transfer 0/1 MB
Email Accounts 0/0
Subdomains 0/0
Parked Domains 0/0
Addon Domains 0/0
Ftp Accounts 0/0
SQL Databases 0/0