I have added the Wyndhamm Power Outage app from the App Catalog, then I need to add my user to the Technicians group but this group doesn't exist.
The Appian Community Edition is on a later version - 24.4 - and the exercise is based on 24.2 version. In Security Summary a warning is detected: "Missing administrator group. No basic users will be able to administer these objects, which includes editing their security or deleting them".
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The way I found to resolve this is to ask for the application to be re-deployed to your environment. To do this, you access the App Catalog from the My Workspace/Explore section of your Community Edition homepage. Search for Wyndhamm app, click on the tile returned from search results. Then in the upper right hand corner of the web page, click on the "Deploy App to Workspace" button. HTH.