Act As Delegate feature is only available in PWA and you cannot open Project Pro when you are acting as delegate. As an alternative solution, you can open Project in PWA and make the necessary changes using below steps.

1. Log on to PWA
2. Start the delegate session.
3. Access Project Center View >> Open the Project in PWA
4. Access the Schedule PDP and Open the Project in Edit mode
5. Edit the Project Plan and Publish the changes.