When using the Agorapulse companion app, there are 2 methods when approving/rejecting posts that have been assigned to you.
Tap the push notification you received asking for your approval
Tap on the Review button
Choose the desired approval action: Approve or Reject.
Please note: you're validating a group of posts and not individual posts
Go to the publishing section
Select the "Assigned to me" filter
Tap on the Review button under the post you want to review
Choose the desired approval action: Approve or Reject