Firstly, you need to ensure the PO has been created in the Purchase Order Entry area.
Secondly, you need to update the assignment with the PO to ensure any new or open timesheets reflect the PO. This is done on the first page of the Assignment Entry:
Finally, if you have timesheets that have already been generated, processed or are mid-process you will need to update these separately.
You can automatically update any un-invoiced timesheets within the assignment, by ticking the "Update all un-invoiced timesheets to use this PO" tickbox when adding the PO, then Save Changes to the assignment.
You can also do this manually on the individual timesheet under Timesheet Display, by selecting the timesheet and updating the PO from the PO drop down. You will also see below the PO that is currently set on the assignment, you can then click 'Update' (the system will also highlight to you in validation if the timesheet PO information differs to that currently held on the assignment):