Click on Edit, go to the Item Category column in your Sales Quotation.  If not previously setup to display in the Items assignment block, go to the pencil and add Item Category to display in the Items assignment block.  Then click on Edit.

Select the drop-down for the appropriate line item, and select a different choice.  If it lists Item (LEIS), change it to Item (DIEN).  If it lists Item (DIEN), change it to Item (LEIS).  Save the quote, and the error should disappear.  I don't know where this error comes from, or why this fixes it.