Invalid Column Name Error but column exist in table

Invalid Column Name Error but column exist in table
alter proc SellingComputation @ItemName nvarchar(50), @InvoiceID nvarchar(50), @Quantity int, @SellingPrice numeric(18,2), @UnitID int, @Discount numeric(18,2) as begin insert into SellingDetails (ItemName,InvoiceID,Quantity,SellingPrice,DiscountPrice,ItemTotal) values (@ItemName,@InvoiceID,@Quantity,@SellingPrice, @UnitID, @SellingPrice-(@Discount/100),(@SellingPrice*@Quantity)- DiscountPrice ) end DiscountPrice is not exist.I want to compute itemTotal.table script CREATE TABLE [dbo].[SellingDetails]( [SDid] [int] IDENTITY(1,1) NOT NULL, [InvoiceID] [nvarchar](50) NOT NULL, [ItemName] [nvarchar](50) NOT NULL, [Quantity] [int] NOT NULL, [SellingPrice] [numeric](18, 2) NOT NULL, [UnitID] [int] NOT NULL, [DiscountPrice] [numeric](18, 2) NULL, [ItemTotal] [numeric](18, 2) NOT NULL, CONSTRAIN ...