1. A concise explanation of the problem you're experiencing.
I'm working through the "Cesium Workshop" tutorial. For the 3D New York buildings, the asset isn't availble.
2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.
var city = viewer.scene.primitives.add(new Cesium.Cesium3DTileset({ url: Cesium.IonResource.fromAssetId(3839) }));
3. Context. Why do you need to do this? We might know a better way to accomplish your goal.
Just learning the library
4. The Cesium version you're using, your operating system and browser.
1.45 (what's specified in tutorial files), Mac OS X High Sierra, Chrome