Search for value and exit macro if found

Try

Sub Move_PO_to_Log()
    Dim POnumber, c
    Dim wbTemp As Workbook
    Set POnumber = Range("M2")
    Set wbTemp = ActiveWorkbook
    ThisWorkbook.Activate
    
    'The code below will search the whole sheet for the POnumber,
    'and then give an error if it doesn't find it.  I really need
    'it to only search Column M, and give a message and exit if
    'it finds the number, or continue if it doesn't.
    Set c = Columns("M").Find(What:=POnumber.value, After:=POnumber, LookAt:=xlWhole)
    If not c is nothing then
        Msgbox "This PO Number is Already in the Tracking Log"
        Exit Sub
    End If