In the approved PO Edit button not available

Purchase Order Approval Workflows have a workflow action that locks the record once the Purchase Order has been approved.

1. On the Purchase Order, navigate to the System Information tab > Active Workflows subtab and check if there is an active workflow on the Purchase Order.
2. Navigate to Customization > Workflows > Workflow and then click on the Purchase Order Approval Workflow used on the Purchase Order.
3. On the State section of the Workflow State, check if there is a Workflow Action that says Lock Record.

The Workflow Action: Lock Record prevents the Purchase Order from being edited once it has transitioned into the said workflow state.