Hi Piyush,
Pls. check your availability check control...May be its checking the replenishment lead time(defined in material master as GR processing time/Planned delivery time) and your PO date of delivery is outside the replenishment lead time...If you don't want to check the Replenishment lead time, pls. check this checkbox in your availability check...
Check availability without replenishment lead time
Indicates whether, during the availability check, the system checks the replenishment lead time.
Use
Items for which the delivery date falls within the replenishment lead time, but for which there is no inventory, are confirmed for delivery after the replenishment lead time. In the case of items for which the delivery date falls after the end of the replenishment lead time, the system assumes that everything can be confirmed.