there is no column named discountPercentage:
//columns
$this->ID = $itemID;//n?
$this->Name = $itemName;//t
$this->Description = $itemDescription;//t
$this->Thumbnail = $itemThumbnail;//t
$this->Weight = $itemWeight;//w
$this->Quantity = $itemQuantity;//n
$this->Price = $itemPrice;//c
//calculations
on the columns click the plus button and add a new column, name it discountPercentage