Yes, there is most likely something wrong with your token. It’s invalid (copy/pasted wrong?), not enabled for the correct APIs, or simply hasn’t had a chance to become fully active yet (Google says that takes 5 minutes, but we’ve seen it take much longer sometimes). Visiting the URL in a web browser and looking at the response can help you figure out what the problem is.
If you’re trying out Cesium for Unreal for the first time, we highly recommend you start with the Cesium for Unreal Samples project. You can install it from the Unreal Marketplace.