In order to use the iCloud generator your account must be an iCloud+ Account, to check whether or not your account is an iCloud+ account you can sign in here: https://www.icloud.com/settings/
If you only have regular iCloud, rest assured iCloud+ plans start at $0.99/month, the difference between plans does not determine how many e-mails can be generated.
The account needs to have an active iCloud email address, to check if you have an iCloud email address that sign in here
If you don’t have an iCloud email setup, this guide will show you how to do so
Note that it is important to differentiate between an iCloud email address and an AppleID.
Here you can see that the iCloud email address is firstname.lastname@example.org and the Apple ID, on the top right, is Jake@aycd.com.
To generate iCloud emails within Profile Builder an iCloud+ e-mail account must be added into the Accounts section.
Once added to the Accounts page, when generating accounts from the Generation page you can select the store to iCloud and begin generating
After 20 or so accounts Apple begins rate limiting the creation, after a few hours Profile Builder will retry generating.