How to Update Stock quantity if I sold items?

I tried to copy and paste it to my original sheet and i changed the name ("Sheet1") to Sheet2 where my work is.
and I changed also Target, Range("B2:B6")) to "D2:D25" in Worksheet_Change() but did not work any ideas?

Function StockLevel(Item_Name As String, Qty_Sold As Double) are these the names of the cells ?

**And if it would be done with a simple formula i will take my chances