Would it work for you to create a View that shows the pending items and then create an Alert based on that view that will send weekly emails?
Based on your comments, I am under the impression that you are trying to create a View using '[PO Date] is equal to 0'. If this is the case, you have a lot to learn about SharePoint I am afraid.
In any case, the filter should probably be '[Status] is equal to Pending' (or something to that effect). I do not see why you included the PO date in the filter (and based on your other question, the calculated field will eventually include the requirement for the PO Date too).
After you create the view, you will need to create the alert based on this view (Send Alerts for these Changes) and opt to receive weekly alert (When to Send Alerts).