Copying array formula to last row using VBA.

Try this...

    With Sheets("SN Validity Check")
        LastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
        .Range("B2").FormulaArray = _
           "=INDEX('Master MICL'!AQ:AQ,MATCH($A2&""ACTIVE"",'Master MICL'!H:H&'Master MICL'!N:N,0))"
        .Range("B2").AutoFill Destination:=Range("B2:B" & LastRow)
    End With