Not able to re-atp 1of the sales order line item

Hi SAP PP Gurus,

When I tried to re-atp 1 of the sales order line item it is giving error as mentioned below. Please let me know where this fix qty indicator in the sales order and how can I de-activate the same. Thanks in advance....

{

Item has no open quantity - Check fixed quantity ID

Message no. V1 601

Diagnosis

The item does not have any open, confirmed quantities and the "fix

quantity" indicator is activated. This means that the customer has

agreed on a zero quantity and nothing is delivered.

System Response

The system issues a corresponding message.

Procedure

If you want the item to receive a confirmed quantity after all which can

be delivered later, you should de-activate the fix quantity indicator.

}