I am trying to set the Markup % for a sales line on a work order task:

The field help states “The markup percentage when the price source is Cost Based”.
I am unable to edit the Price Source field for the line. The field help states “Cost based: The price has been calculated using the Markup %”.
Feels like a bit of a chicken / egg scenario, is there somewhere I have missed a prerequisite in the Work order that is not allowing me to do edit this field?