A leg tab can't be dragged

What you see. Dragging a leg tab does nothing and the graph says “Load the chain for this expiration to move strikes”.

Why. The chain for that expiration is not loaded, so there are no listed strikes to snap to.

What to do. Load the chain for that expiration. Meanwhile the stepper still works: click the tab for − / +. Stepping a 1-lot to zero removes its tab; to go through zero, add the opposite leg in the chain. See Edit legs on the curve.