To enable the Approval Workflow feature, you must:
Click on the OKRs drop-down menu > Manage sessions
Click the Create a session button
Follow through the steps and create a session
Navigate to the Workflow section
Choose how many people you want to approve every OKR in this session
Click Use workflow
Click Create session
Only account administrators and custom-made roles can create a new session or edit an existing one. Thus, they're the only ones who can enable the Approval Workflow feature.
If account administrators enable the feature, they can't go back and disable it. That's why, we've added a confirmation state to inform the session creator that there is no going back to "Don't use" after the session is created with a Workflow enabled.