@nadicad, Is the appRole "Viewer" been assigned to the user?
Ideally when you add an appRole using the App Manifest, that role also has to be assigned to the users who would be accessing that app.
Make sure you the users have that role assigned to them.
You can refer to the following article for reference: https://learn.microsoft.com/en-us/azure/active-directory/develop/howto-add-app-roles-in-azure-ad-apps