Hi There we are using Odoo 11 Community Version. 

The setup: 6 Work center to finish final product. Last one is Packing, now all Work center completed their work orders properly (Lot numbers for the Products and Materials used are used) but now at the last step putting the final product into Carton, the Work center cannot finish the process. and gets

No puede completarse la operación, probablemente debido a lo siguiente:
- eliminación: es posible que esté intentando eliminar un registro mientras que otros registros todavía tienen referencias a él 
(I guess this one does not apply as we do not delete any record)
- creación/actualización: un campo obligatorio no se ha introducido correctamente
[objeto con referencia: product_uom_id - product.uom.id]  (missing mandatory field) 

I am new to Odoo and have checked Google for answers and the forum but cannot get any further. Revised all options where Unit of Measure can be modified, no success

thanks in advance!