I just had a web chat with Google support with regards to using the google home with voice commands to access music, and they stated you cant do it (in the UK), without a subscription.
This is much like the issue I am experiencing with my Pixel phone (accessing music via the assistant) and can only assume google have changed their access and subscription policies recently.
Our family enjoys the Google Play Music Family Plan. We have had it for nearly two years and think it a great service. Up to 5 family members enjoy the Plan and we all chip in and pay in total £14.99 per month.
https://support.google.com/googleplaymusic/answer/6317786?hl=en
and
https://www.hotukdeals.com/deals/google-play-music-family-plan-now-available-1499-pm-2347300