Birendra,
You have to perform an availability check AFTER the appropriate supply element is created.
During Sales order creation; and prior to saving; there is no planned order created (possible exception of Assemble to Order). Therefore, during sales order creation, the standard ATP results is 'unconfirmed' unless you are including 'lead time'.
Later, you use whatever means that exists in your business scenario to create a supply element. If the supply element is adequate to increase the ATP qty to > sales order requirements quantity, then you COULD perform another ATP check. During this check, you MAY be able to confirm the sales order.
Once the original sales order is saved during creation, in general, it requires another ATP check to confirm the sales order. In MTO scenarios, any supply elements MUST be relevant to the Sales Order/Item of the Sales order. Other 'firm planned orders' will not be usable to increase the ATP quantity.
Any questions you may have about 'why' for ATP can be answered in CO09.
Best Regards,
DB49