I have a form "packing list" and "invoice". "Invoice" has a lookup field which is "PL-[packingListNumber]" as you can see in the image. What I need is to propagate an edit from packing list to its related invoice. Thus, at "On Edit - On Success" I wrote: 

invoice = frmInvoice[Packing_List = input.ID];

// my edit...

When I try to edit my packing list, I've got an error which says that is unable to update "invoice".

The ID columns of Packing list and Invoice form are visible.

Does anyone know how can I fix this?

Many thanks!