Before you're able to create an account in Web Host Manager (for your customers) you need to first create a package for the account to use.
You can do this by logging into Web Host Manager and clicking Add a Package:
Once on the creation screen you need to define all the details to your hosting package:
As you can see, I start with a name for my package and then define the limits I want for my customers.
It's important to note, if you want to offer unlimited disk space and bandwidth for the package, you must enter 99999999 vs. choosing Unlimited. This is unfortunately the way the Reseller System is designed in Web Host Manager.
We do allow overselling but if you select Unlimited the package will not function. If you have a VPS, you can use the unlimited option.
Even if you do plan on using WHMCS to create accounts, you'll still need to setup your packages here first.