Before you can use an object  variable like Wbook, you have to use the SET
command to point it to a real object. For example:

Set Wbook = ActiveWorkbook

However, I'm not sure you need an object variable  in this case. Excel VBA has
a predefined object called ActiveWorkbook which always refers to the open
workbook which is currently active (to be sure it's the active workbook, click
in any cell in the workbook just before running your macro). Using
ActiveWorkbook the code  might look like this:

Sub testing()
Dim i As Integer
For i% = 1 To ActiveWorkbook.Sheets.Count
If LCase(Left(Sheets(i%).Name, 4)) = "exec" Then
Sheets(i).PrintOut Copies:=1
End If
Next i%
End Sub