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My answer is below...

The issue is in the landmarkData.json file, the problem comes from the tutorial page. If you see, landmarks[0] gets you imageName = turtlerock Now, if you go to Assets and see the name of the file it's named turtleRock, with capital R. If you go to the JSON file and change the naming to turtleRock your preview will work. For every other object on that JSON file, it works correctly