This issue has been identified as a bug and will be fixed in future release of Tally.ERP 9.
In the interim you can set the option Complete Accounting Allocations in Order / Delivery Note? option is set as Yes.
This error message will appear only in case above option is to No.
In case this option is set to Yes then this error message will not appear and voucher can be saved.
This option is available in F12 Configuration of Purchase order/Sales Order/ Delivery Note/ Receipt Note voucher entry.
This option also available in Gateway of Tally > Press F12 > Invoice / Order Entry.