Hi Mr. John,
Your are undergoing through half process of budgeting Functionally.
Creating Original budget by CJ30 is not sufficient to activate the Availability Control on WBS.
You have to follow Budget release by CJ32 once you create original budget by CJ30.
When you create budget by CJ30, WBS system status changes to BUDG. It Mean that Budget is applied by the user for approval from management for further utilization.
Once budget get released by Management through CJ32, WBS system status changes to AVAC (i.e. Availability Control Activated) It mean Management approves your budget for utilization.
Once you done CJ32, all the commitment will be considered against budget availability. And as you have "0" zero budget system will not allow to make any PO.
Thanks
Dnyaneshwar