Here is the Error message screenshot
If you see the above screenshot, when you are signing into duo using Turning Point App or authenticating to Duo-protected apps on iOS 11.3 or later, then follow the instruction and “Approve” as stated below.
“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.”
Please visit the link for detail: https://help.duo.com/s/article/4344?language=en_US