Can't Insert reference operation set to Production Order

Hi experts,

I'm trying to insert Reference Operation set to production order but can't !

Steps:

1-CA11 --- Create Reference Operation Set --- UOM (KG)

2-CO02--- Edit--- Insert--- Reference Operation Set (Order Quantity UOM "M")

I got error message "The reference operation set XXXXX you selected is not valid 'Message No. C2224"

Thanks,