03-12-2019 12:45 AM - edited 03-12-2019 12:47 AM
I had the same problem on a Moto Z Play, XT1635-02. Running Android 8.0.0.
Input language in the Google Assistant settings was set to supported language, English UK. But still didn't work.
Then I noticed that Android system > languages and input > Languages was set to English, but region Switzerland. Removed that and added English, region UK.
After a restart, OK Google, voice match and voice model training worked again.
So make sure that all voice settings, in Google Assistant and Android System, are set to a supported language, including the region.