If Make Inventory Available is Disabled on the Source Location, Serial Number on Hand is not Available in the Item Fulfillment of a Transfer Order.

As from the Help Documentation (Topic: Creating Locations), check the Make Inventory Available box if inventory at this location can be stocked and can be committed on sales or fulfillments. Clear this box if inventory at this location can be stocked, but cannot be committed on sales or fulfillments. Clear this box if inventory at this location can be stocked, but cannot be committed on sales or fulfillments.

Transferring Serialized or Lot Numbered Items using Transfer Order (Transactions > Inventory > Enter Transfer Orders) requires fulfillment of the serialized and lot numbered items. Serials and Lot Numbers should be available and committed to the Transfer Order before it can be fulfilled. However, such will not be available if Source Location's Make Inventory Available is disabled.
 
Answer/Solution:
 
Use the Inventory Transfer to transfer inventory from one location to another. Navigate to Transactions > Inventory > Transfer Inventory to go to this page.

This basic inventory transfer decreases items in the source location and increases them in the receiving location, all in one step.

Enhancement Request# 219267, "Setup > Company > Location > Make Inventory Available = F> Transfer Order Fulfillment should still make inventory available to be transferred to another location." is also logged for this.