Your Chromebook uses the ExpressVPN app for Android, which contains all the features of ExpressVPN. You do not need a VPN browser extension to use ExpressVPN on your Chromebook. Just install ExpressVPN for Android, and you’re good to go.
That said, every ExpressVPN subscription does include access to our browser extensions for Chrome or Firefox. These entirely optional add-ons work in partnership with the ExpressVPN apps for Windows, Mac, and Linux. They do not work with Chrome OS, but Chromebook users enjoy full VPN protection without any extension.