From a quick look at the console, the error message is Cesium.createWorldTerrain is not a function
and in code, there is
var viewer = new Cesium.Viewer('cesiumContainer',{
terrainProvider: Cesium.createWorldTerrain()
});
The createWorldTerrain function was replaced a few years ago. (Looking at the date in the URL, it’s not surprising that the code does not work with the latest version (1.138) of Cesium - it might be that the code is 10 years old, and a lot has changed in the meantime…)