I would do the calculation as TotalQuantityOnHnad - (IM_ItemWarehouse.QuantityOnSalesOrder+IM_ItemWarehouse.QuantityOnWorkOrder)

The lookup in Item Maintenance/Inquiry can only read CI_Item or other linked tables where there is a one to one relationship between records.  Since IM_ItemWarehouse contains multiple records, you normally can't read fields from it in this lookup.

You can however use the info in this thread, https://www.sagecity.com/support_communities/sage100_erp/f/sage-100-personalization-customization-and-productivity-tools/125980/uds-on-im_itemwarehouse-table_post-write-doesn-t-get-triggered/336002#336002, to set up a link on the item code and default warehouse code from CI_Item so you can read from just that record in IM_ItemWarehouse.

I can't say how successful this solution would be, since I cannot click the "Link" button in Export Job Maintenance. 

Assuming "TotalQuantityOnHnad" was a typo, implementing this solution as intended does not seem to work. It still just outputs the total quantity on hand. 

This won't work without first creating the link as outlined in my other answer.

You need to enter or select a job name and then enter or select a table.  I would suggest using CI_Item as the table so it is prepopulated when you click the link button.