Error: OAUTH2 SERVER LIBRARY Unavailable
I encountered the same problem, being new to Drupal 8 development, and this project currently lacking documentation for its use with Drupal 8, probably because it is currently only available through a "dev" edition.

The solution I used was to add the two oauth2-server-php Composer dependencies in the "composer.json" file at the root of the Drupal folder, which are currently:

"require": {
    "bshaffer/oauth2-server-php": "~1.7",
    "bshaffer/oauth2-server-httpfoundation-bridge": "~1.1"
}


Then install them with Composer by executing "composer install". 

You can reach the project issue where I got help there: https://www.drupal.org/node/2897289#comment-12202222