1. A concise explanation of the problem you're experiencing.
Running the example at https://cesium.com/docs/tutorials/getting-started/#your-first-app
It references a script at https://cesiumjs.org/releases/1.54/Build/Cesium/Cesium.js which is a 404.
2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.
It's on your website.
3. Context. Why do you need to do this? We might know a better way to accomplish your goal.
I'm trying to see if Cesium is worth my time. The initial example doesn't even work. So I'm leaning toward no...
4. The Cesium version you're using, your operating system and browser.
Allegedly Cesium 1.54 but it's not loading because there's a 404 from your website.
MacOS 10.14.2 Firefox 65.0 (latest)