VBA Error 400

I am new to writing VBA scripts and I am having problems getting my scripts to run. My code is:

Sub Cellborder()

'Formats all cells, below the row with column headings, that have data with cell borders

Dim nLastRow    As Long
Dim nLastColumn As Long
Dim nLastCell   As Long

nLastRow = ActiveSheet.UsedRange.Rows.Count
nLastCol = ActiveSheet.UsedRange.Columns.Count
nLastCell = ActiveSheet.Cells(nLastRow, nLastCol)

    ActiveSheet.Range("A7").SpecialCells(xlCellTypeLastCell).Select
    Selection.Borders(xlDiagonalDown).LineStyle = xlNone
    Selection.Borders(xlDiagonalUp).LineStyle = xlNone
    With Selection.Borders(xlEdgeLeft)
        .LineStyle = xlContinuous
        .Weight = xlThin
        .ColorIndex = xlAutomatic
    End With
    With Selection.Borders(xlEdgeTop)
        .LineStyle = xlContinuous
        .Weight = xlMedium
        .ColorIndex = xlAutomatic
    End With
    With Selection.Borders(xlEdgeBottom)
        .LineStyle = xlContinuous
        .Weight = xlThin
        .ColorIndex = xlAutomatic
    End With
    With Selection.Borders(xlEdgeRight)
        .LineStyle = xlContinuous
        .Weight = xlThin
        .ColorIndex = xlAutomatic
    End With
    With Selection.Borders(xlInsideVertical)
        .LineStyle = xlContinuous
        .Weight = xlThin
        .ColorIndex = xlAutomatic
    End With
    With Selection.Borders(xlInsideHorizontal)
        .LineStyle = xlContinuous
        .Weight = xlThin
        .ColorIndex = xlAutomatic
    End With
End Sub

When ever I try to run it, it comes up with a error 400. What could the problem be?

Try this:

Sub Cellborder()
    With ActiveSheet.UsedRange
        With .Offset(1).Resize(.Rows.Count - 1).Borders
            .LineStyle = xlContinuous
            .Weight = xlThin
            .ColorIndex = xlAutomatic
        End With
    End With
End Sub