VBA to fill formula down till last row in column

I want excel to draw down this formula so in cell M4 it would be =G4&”,”&L4 Cell M5 would be =G5&”,”&L5 and so on.

My code looks like:

Range("$M$3").Formula = Range("G3") & (",") & Range("L3")

Dim Lastrow As Long

Application.ScreenUpdating = False

Lastrow = Range("L" & Rows.Count).End(xlUp).Row
Range("M4").FormulaR1C1 = Range("G4") & (",") & Range("L4")
Range("M4").AutoFill Destination:=Range("M4:M" & Lastrow)
ActiveSheet.AutoFilterMode = False
Application.ScreenUpdating = True

VBA To Add Formula To Cell

I am attempting to write some VBA which will add header text to 3 cells then fill a formula all the way down to the last row. I have written the below, which writes the headers no problems, but when it get’s to my first .Formula it throws a

Application Defined or Object Defined error

What needs to be altered so that this macro will execute successfully? (The formulas were pulled directly from the formula in the cell, so I know they are valid formulas at least on the “front-end”)

Function Gre()
    Range("E2").Select
    ActiveCell.FormulaR1C1 = "Under"
    Range("F2").Select
    ActiveCell.FormulaR1C1 = "Over"
    Range("G2").Select
    ActiveCell.FormulaR1C1 = "Result"

    With Range("E2:E" & Cells(Rows.Count, "C").End(xlUp).Row)
        .Formula = "=IF(C2<B2,B2-C2,"")"
    End With
    With Range("F2:F" & Cells(Rows.Count, "C").End(xlUp).Row)
        .Formula = "=IF(C2>B2,C2-B2,0)"
    End With
    With Range("G2:G" & Cells(Rows.Count, "C").End(xlUp).Row)
        .Formula = "=IF(F2>0,'Issue',"")"
    End With
End Function