I might have manually "forced it" to appear, or it may have just coincidentally started working after I did something. So I offer this as my experience only, and it may or may not help anyone else.
I have an HTC 10 running Nougat 7.0. I set my language to English US and restarted the phone but nothing seemed to change. Long press on the home key or opening the Google app didn't do anything new.
Then, in the Google app, I pressed hamburger menu (top left) and selected Settings, then Voice. About the second item in the list said:
"Ok Google" detection
Talk to the Google Assistant to get things done
I thought "Well, it knows about Google Assistent" so I clicked on that list item and it gave me some options (e.g. trusted voice) and went through the voice training (basically saying "OK Google" three times).
After doing these things, I closed the Google app and long pressed the home button again and the Assistant popped up.