Service items can only be backordered if the items are set to Allow Backorders on the Item Card, and if the Sales Document ID has a Backorder ID set.

Allowing the item to be backordered:

1. In Inventory Lookup, navigate to the Proprties tab of the item and click the Item Number ellipses to open up Item Maintenance (see also: Item Maintenance)
2. Here you may check the Allow Backorders checkbox and click Save.

Allowing the items on the Sales Document ID to be backordered:

1. In Dynamics GP, open Sales Order Processing (Microsoft Dynamics GP button >> Tools >> Setup >> Sales >> Sales Order Processing)
2. Click Sales Document Setup on the lower left and select your Sales Document Type. This will open the Sales Order Setup window
3. In the Sales Order Setup window, select your Order ID, check "Transfer Order to Back Order" and set a Back Order ID in the Use Back Order ID field. Save