If you see a "Your session expired. Please try again." error when attempting to authenticate via Duo into a mobile app (like Canvas) & you are running iOS 11.3 or newer, this is due to a change Apple made to the iOS starting with this version. Duo hopes to have a permanent fix available soon.
As an interim workaround, when you receive the Duo Push notification, perform a "force touch" (press hard on the notification and hold) to show the "Approve" and "Deny" actions instead of tapping it to open the Duo Mobile app. You can also drag down on the notification to view these options.
Alternatively, you can enter a passcode or perform phone call authentication to avoid switching into the Duo Mobile app, if your organization's Duo application policy permits those factors.
For more details, see Duo's support page on this issue by clicking here.