You want bidder to add new line items but you do't see any option to add new items in interactive form
SAP Business ByDesign
This check box is not applicable for interactive form. but its used if the quote is created via webservices.
This is expected system behavior and thus you may
ask your partner for help
ask your partner to build webservices
add all new requirements on Customer Influence Forum at https://influence.sap.com/sap/ino/#/campaign/886
How to add items, bidders cannot add items , KBA , AP-RFQ-RFQ , Request for Quote , Problem