Unable update 'Material Stored' field in the work confirmation creation

I am creating a work corfirmation and I can see the 'Material Store' field, however I can not update the field.

In the iSupplier user guide I have checked:

Note: The Material Stored field allows the supplier to request a

payment for material which has been purchased and stored on

site for use. Using the work confirmation, the supplier can

enter the requested amount based on work actually completed,

and has an option for requesting payment for material

purchased but not yet consumed in work.

How Can I update this field? Which is the funcionality for this field?

I have the same problem from iProcurement, Purchasing and iSupplier Portal.