By default, Purchase Order Matching is enabled in the Bill Creation and Approval workflow settings.
To double-check, go to Approval workflows, open the Bill workflow and click on the Gear icon. Then make sure that the option "Allow users to match Bills to Purchase Orders" is activated.
Next, decide on the advanced setting:
Can a Bill be approved if it is not matched to a Purchase Order?
This setting regulates whether or not Bills can be approved if one or more lines are not matched to their corresponding Purchase Order(s).