Microsoft will be retiring Office Mix on 1st May 2018. Office Mix was an add-on for Microsoft PowerPoint which allowed you to create quizzes, apps and polls within a presentation.
If you used Office Mix you may have uploaded some work to the Office Mix Website (https://mix.office.com). You will need to log onto this site and download your Mixes as PowerPoint files or “Migrate” them to Microsoft Stream. To do this log onto the site and click “My Mixes”. You then get the option to migrate to Stream or download them.
You can find out more about this on Microsoft’s Uservoice Website.