I downloaded Google Assistant on my Find X from play store and it shows “The Google Assistant isn’t available on this device.” Took me a while to fix it.
Make sure you have the newest version of Google Service Framework(usually you do) and check if you have Google Partner Setup installed(https://www.apkmirror.com/apk/google-inc/google-partner-setup/).
Now go to settings and change your system language to French. After switching language go to google assistant and enable it. The process should be straightforward even you don’t understand French. After enabling the assistant switch back to English. Now you should be all set.