iOS has configurable content restrictions that can potentially prevent the Duo Prompt from displaying correctly.
If you see a gray box or “session has expired” error instead of the Duo Prompt on your iPhone or iPad, please check whether you have content restrictions enabled or Disable the restrictions.
If you see the above error message, then please follow one of the following instructions to resolve the issue:
This can be found on the device by going to Settings --> General --> Restrictions --> Websites.
First, check whether you have content restrictions enabled or disabled by going to Settings -->General -->Restrictions -->Websites.
Option 1: If you do not want to disable the content restrictions entirely, you can add duosecurity.com to the Whitelist within the content restriction page on the iOS device.
Option 2: To completely disable content restrictions, uncheck “Limit Adult Content” and change it to “All websites.”
Option 3: Settings --> General -- > Restrictions --> disable the Restrictions
Here is the link https://help.duo.com/s/article/3710?language=en_US