solve run time error

The following code has to search a column to determine if first letter is a number. if not then replace cells preceeded by O,o,l and blanks by 0,0,1 and blank space removed respectively. and delete other cells.
it is giving run time error 424: object required.
how do i make this code to search the entire worksheet? and also remove space in between numbers??

Sub Testing1()
Dim i As Integer
Dim ch As Characters


For Each num In Range("A2:A10")
 Set ch = Left(num, 1)
 If Not (IsNumeric(ch)) Then
     num.Font.ColorIndex = 11
     num.Interior.ColorIndex = 2
     
  Select Case ch
    Case "O", "o"
      left(num,1)= "0"
       num.Interior.ColorIndex = 3
          
    Case "l"
        left(num,1)= "1"
       num.Interior.ColorIndex = 3
    
    Case " "
       left(num,1)= ""
       num.Interior.ColorIndex = 3
    
  End Select
  End If
Next num
End Sub