I need to use A as the reference, because H in this case is an added column, so it's cells won't be filled with anything. For all data rows, there will be data in column A.

When I run the code, I get "Run-time error '1004': Autofill method of Range class failed" and then when I click debug, it highlights the line with
Code:
Range("D2").AutoFill Destination:=Range("D2:D" & Cells(Rows.Count, "D").End(xlUp).Row), Type:=xlFillDefault
(This is from a testing example, but the "Cells(Rows.Count, "D").End(xlUp).Row" part is the same.