Hi Ludovico,
From my point of view, if you want to create one QP reference to the header material of production order, the complaint quantity must be delivered within the production order already. Otherwise you will get the information complaint quantity is difference.
Anyway on the pop-up information windows, you can try to click the button "Enter". If it's not error message, you can go to next screen and complete this notification creation.
Regards
Tao