The WebXR project from Mozilla wasn't working with Unity 2021 when this chapter was being written (it may have been updated since). If you need to use the official Mozilla project from 2020, then you can develop with Unity 2019.4.13f1 LTS.
Since the project is open source, some members of the community are working to help update the WebXR resources themselves. One version of the Mozilla Desert demo project that's kept up to date is from De-Panther. It can be downloaded as a ZIP file from https://github.com/De-Panther/unity-webxr-export:
If you just want to install the WebXR Export package (not the whole demo project), OpenUPM has published an up-to-date package on GitHub that you can use by adding a package to Package Manager via https://github.com/De-Panther/unity-webxr-export.git?path...
