Unable to do OSP for Steel Plate Items as Steel Plate UOM is different than OSP Labour Charges

Summary:

We are trying to implement OSP Process for Steel Operation; where Steel Coils are getting Cut into different Sizes Steel Plates by outside Vendors.

Those Steel Plates we have defined Primary UOM as Number.

Where as the Outside Vendor charges the Steel Processing in KG or Metric Tons.

So I have created a Separate Item for Steel Plate (Number as Primary UOM) & a separate Item for Labour Charges for OSP Operation (KG/MT as Primary UOM)

And, post defining the BOM when I was trying to create the Standard Operation for OSP Work Order. I was getting the following Error -