Error: Line#1 This line references a source document or source document line that has expired.

When the PO does not go to approval and gives below error:

This error occurs because effective date of Contract Purchase Agreement (CPA) (43918) has been expired which is specified in PO.

Go inside Contract Purchase Agreement (43918) > Terms and Conditions>and change effective date of CPA.

Note that, CPA should be approved before you create PO.  So after changing effective end date, send CPA  to approval again and then send PO 44642 to approval as CPA is approved.