It's bad practice to use ActiveWorkbook
when you don't need to. It's always better to set your workbooks and worksheets to actual variables that you can call on. I think your code is activating another workbook then trying to select a range in a worksheet it can't find.
Sub TryThis()
Dim wbk As Workbook
Dim ws As Worksheet
Set wbk = Workbooks("myWorkbook.xlsm")
Set ws = wbk.Worksheets("Sheet1")
'Now when we say "ws." it is actually "Workbooks("myWorkbook.xlsm").Worksheets("Sheet1")."
'This is okay to start with but it's better to work with the cells directly
ws.Select
Range("N2").Select
Selection = "myText"
'This is much faster and you won't have to worry about what is currently selected
ws.Range("N2") = "myText"
End Sub