I know this is a server-side thing. But, I had to update my Google Play Services app, just to try. This app is not in the Play Store but is listed in your list of apps in settings. I was on ver. 10.2.98 and I side-loaded 10.2.99. I used the Play Services Info App and downloaded the update from the apk mirror.
Bottom line, after the update and a phone reboot... Still no Google Assistant. So, I/we will just have to wait until Google flips the switch to route our individual Google Now inquiries to the Google Assistant servers.
I figured this attempt was futile, but I had to try it, and wanted to share the result with those of you who are similarly inclined.