Not able to create valuation- category and type for existing material

Dear,

i have the following issue. I've got a material for which i want to create a couple of valuation types, however this is an existing material without valuation category. Now as i entered the valuation category in order to create a valuation type i got an error message. I know the material cannot have any stock or open items on po's etc.. However the error log shows that i cannot extend the material because of open PO's. These PO's have the final delivery and final invoice indicators ticked so i don't understand why i get this message. I deleted the PO lines and now the valuation category could be created. However i don't want to delete all PO's which were created in the past for these materials. Do i miss something here?