There are number of reasons for why the Discover feature will not show up:
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The Discover feature works for Microsoft 365 commercial accounts and on-prem accounts with modern auth enabled. It will only be displayed if the Outlook mobile app is configured with at least one commercial account.
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You may not see the Discover feature if your company administrator has enabled control of this feature through the item Insights setting.
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The Discover feature will not show if the tenant administrator has turned off the Discover feature through MDM.
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The Discover feature tries to find documents that are interesting to you by looking at what users you collaborate with are doing. If users close to you have not been collaborating on documents recently, there will be nothing to show and the Discover feature will not be displayed.