inserting comments to cells based on the values of that cell and one other cell (VBA)

Try using this Worksheet_Change event procedure

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim rCell As Range, r1 As Range, r2 As Range

    If Not Intersect(Target, Range("D29:AP29")) Is Nothing Then
        Set r1 = Range("D12:AP12")
        Set r2 = Range("D29:AP29")
        r2.ClearComments

        For Each rCell In r2
            If rCell <> 0 Or r1.Cells(1, rCell.Column - 3) <> 0 Then rCell.AddComment ("error")
        Next rCell
    End If
End Sub