How to show fields in "Sales Orders" in invoice template.

Hi,

Sales Orders: SO0001

Filed: SOF_A

Filed: SOF_B

New a invoice INV0001 from SO0001, so INV0001 is connected with SO0001

Print Invoice INV0001 from invoice template.

How can I show SO0001's SOF_A, SOF_B in my invoice, as I can't select Sales Order in the pull-down menu of "Available Merge Fields"?