Form Extension/Orch, Define Output section says "Output mapping for the event is not allowed"?

I want to add an orchestration to a grid column, it's a simple orchestration that removes leading spaces, however when I get to the Define Outputs section it says "Output mapping for the event is not allowed". It allows output mapping when the field being mapped is in the header.

Does anyone know why it won't allow output mapping in the grid? I attached the orchestration setup for the form extension. The form extension is Multi voucher-Single Supplier P041017_W041017A

Thank you.