Resolve "Authentication Required" Error While Accessing An App

When you're trying to access an app and receive "Authentication Required" popup in every click then you need to understand whats going on.
The apps and SharePoint Portal are different environments. Two cannot work in same DNS and AppDomain, you need to configure Apps to have different DNS and AppDomain by using Powershell when adding App Support to your SharePoint Portal.
Because of this AppDomain issue, you're receiving the loopback popup window named "Authentication Required".
To solve this, there's a workaround many people use: Regedit entry.
  1. Click Windows Key + R and enter regedit or simply run regedit (all the same).

  2. Locate HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa.

  3. Create a new DWORD 32bit named DisableLoopbackCheck.

  4. Assign its value as "1".

  5. Reboot.

  6. Open the app again.
That should work and solve your issue.

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