Mac OS and iOS have content restrictions that may cause issues with the way the Duo Prompt renders - it will typically show a grey box with no options instead of the Duo Prompt.
Note: This information can be found at https://help.duo.com/s/article/3710?language=en_US.
iOS 12 or Newer
Navigate to Settings > Screen Time > Content & Privacy Restrictions > Content Restrictions > Web Content.
Uncheck Limit Adult Websites to completely disable content restrictions.
Note: If you do not want to fully disable restrictions, you can add duosecurity.com at the bottom of the "Allowed Websites Only" list.
iOS 11 or Older
Go to Settings > General > Restrictions > Websites.
Uncheck Limit Adult Content to disable all content restrictions.
Mac OS 10.15 (Catalina) or Newer
Open System Preferences.
Navigate to Screen Time > Preferences > Content & Privacy.
Set any Web Content restrictions to Unrestricted Access.
Note: If you do not want to fully disable content restrictions, you can add duosecurity.com in the "Customize" section of the "Allowed Websites Only" option.