As the title suggests, what should I do?
Unfortunately, WebGL is currently not supported. This is due to the fact that Cesium for Unity relies on native code interacting with the Cesium Native library, and this native code can’t be easily ported to the web. Creating a WebAssembly build of Cesium Native to allow it to be used with web platforms is something we’ve had our eye on for a while, but it’s not currently on the short-term roadmap. However, if you’re interested and have the time to contribute, investigating WebAssembly builds of Cesium Native and making pull requests to bring this goal closer to fruition would be welcomed!