Tablets are not going to receive Google Assistant according to a confirmation from Google. While the voice activated digital assistant initially launched with the Pixel and the Pixel XL, as well as Google Home, Google eventually made it clear they would be rolling the software out to other devices that were running on Android 6.0 Marshmallow. This unfortunately only includes smartphones that are running on Marshmallow and above and leaves out tablets. The downside is that Google doesn't seem to have given a reason as to why tablets aren't included, but whatever their reasoning may be it seems that you'll simply have to rely on your phone for all of the interaction.
With that said, Google didn't just leave out any explanation as to why tablets won't get the software feature, but they also didn't confirm that tablets would "never" get it. This means there is always the possibility that they could be working on a solution to allow this for tablets and may simply be holding off on including them for now, though it's important to remember that their decision to leave tablets off the list of supported device may actually be a final one and it may not change in the future.
Google initially started rolling Google Assistant out to additional smartphones running on Marshmallow and Nougat as of earlier this month, though not all users have necessarily seen it show up just yet. If you're unsure, you can check to see if it's been tossed to your device by holding down on the home button and long pressing it, which should bring it up if it's there. For any users who do already have it or have had it since the launch of the Pixel and Pixel XL, it seems that Google may be adding a new feature to Google Assistant soon, which is the arrival of screen search button that appears to be in limited testing. If you also prefer to have Now on Tap show up when long pressing the home button, there is a way to revert it back to that even on the Pixel devices, and it seems to be a pretty easy process.