OE Inserting New Line Error (Incorrect Procedure. Tax Group must be entered before inserting a detail line)

Hi guys,

I'm inserting new detail line for the existing Order. and I hit this error: Incorrect Procedure. Tax Group must be entered before inserting a detail line. Location, Item, Qty. Ordered, Price List, Expected Date, Unit Price, Tax Included, Tax Authority, Tax Class, Tax Base, Tax Rate & Tax Amount fields are already added.

Any idea..?

Thanks in advance!