Simple Workflow

“Bill of Marterials” is a standard field on the Work Order.   I need to make this field mandatory but it is not available to check when I edit the form.   Okay, so I figured I would just create a simple workflow to achieve this.

I created a workflow with Record Type = Transaction and Sub Type = Work Order.   The problem is that when I created a new action and selected “Make Field Mandatory”, the Bill of Materials field was not available to select under Parameters >> Field.

Am I missing something?