3014150 - Field 'Open Quantity' cannot be modified in VF01 app when creating billing document with reference to a delivery note - SAP S/4HANA Cloud

When creating a billing document with reference to a delivery note in VF01 app, after clicking 'Selection List', it's found the field 'Open Quantity' is greyed out thus it cannot be modified.

• SAP S/4HANA Cloud

1. Open VF01 app.
2. Fill in a delivery note number as the document to be processed.
3. Click 'Selection List'.
4. The field 'Open Quantity' is greyed out thus the billable quantity cannot be modified. 

The unexpected billing item has an item category whose billing relevance is neither 'O' nor 'K'.

Use item category whose billing relevance is 'O' or 'K' so that the open quantity will be modified.

Item Category, Billing Relevance , KBA , SD-BIL-IV , Processing Billing Documents , Problem