You need to receive that lot number into inventory.. You can do it one of two ways:
First, make sure the Costing method for this inventory item is set to "Lot" (Main folder-tab of I/M Maintenance).
(1) Inventory Transaction Data Entry (I/M main menu). Select receipt-type transaction. When finished entering the lot #'s, their quantities and costs, run the I/M Transaction Journal (sometimes referred to as a register) and choose Yes to update.
(2) Purchase Order Data Entry / Receipt of Goods Data Entry. If you have the P/O module you can enter a purchase order for the item. You can then go to Receipt of Goods Data Entry and enter a receipt of goods against the original purchase order. In Receipt of Goods entry, at the line level, when you enter a quantity for an item that has a costing method of "Lot/Serial" and hit the Ok button, it will popup a distribution window where you can enter each lot # and corresponding quantity. Run the Receipt of Goods Register and click Yes to update to complete the process.