After Creation of new trial version of 365 CE, I have seen a message on the app home page

“You are using apps designed for the legacy web client. For best results, update your apps to Unified Interface.”

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Similar notification will be visible to System Administrators whenever they use an app designed for the legacy web client, as shown below:

“This app is designed for the legacy web client and might have features or customizations that aren’t supported in Unified Interface. For best results, update it to Unified Interface.”

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The following are recommended ways to update the apps to Unified Interface based on how the apps were installed in the environment.

Apps created in your Sandbox environment

  • Be sure to import the changes in your target environment via a managed solution only. See Import, update, and export solutions for guidance on installing an update to an existing managed solution.
  • Modify the app properties by following the steps detailed in Manage app properties, and set the Client type to Unified Interface
  • Import the changes to your target environment via a managed solution update.

Apps installed from AppSource

  • Contact the app publisher and get a new version that updates the apps to Unified Interface.

Apps obtained from an ISV or any other third party publisher

  • Contact the ISV (Independent Software Vendor) or the third-party app publisher and get a new version that updates the apps to Unified Interface.

In my case, as I created a brand new trial environment, the solution gets resolved by following this link

https://docs.microsoft.com/en-us/powerapps/maker/model-driven-apps/manage-app-properties

Reference Links:

https://docs.microsoft.com/en-us/dynamics365/customer-engagement/admin/update-apps-to-unified-interface

https://docs.microsoft.com/en-us/powerapps/maker/model-driven-apps/manage-app-properties