You can create from Automated actions .. settings>technical>actions while on developer mode. If you don't view this menu, you would have to install from Apps 'Automated Action Rules' module.

Create new:

Model Purchase Order
Trigger - On Update
Trigger Fields - Status (purchase.order)
Apply on - Status '=' Purchase Order
Action to do - Create Next Activity

You can then select the activity to be created and whom to be assigned and Due Date