Module not found via Composer
It doesn't say the module was not found, it says there is no compatible version.

My composer version gives a much better error:

$ composer require "drupal/simple_oauth:^2.0-alpha4"
./composer.json has been updated
&gt; DrupalProject\composer\ScriptHandler::checkComposerVersion
Loading composer repositories with package information
Updating dependencies (including require-dev)                          
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Conclusion: remove drupal/core 8.2.3
    - Conclusion: don't install drupal/core 8.2.3
    - drupal/simple_oauth 2.x-dev requires drupal/core ^8.3 -&gt; satisfiable by drupal/core[8.3.x-dev, 8.4.x-dev].
    - drupal/simple_oauth 2.0.0-alpha4 requires drupal/core ^8.3 -&gt; satisfiable by drupal/core[8.3.x-dev, 8.4.x-dev].
    - Can only install one of: drupal/core[8.3.x-dev, 8.2.3].
    - Can only install one of: drupal/core[8.4.x-dev, 8.2.3].
    - Installation request for drupal/core (locked at 8.2.3, required as ~8.2.0) -&gt; satisfiable by drupal/core[8.2.3].
    - Installation request for drupal/simple_oauth ^2.0-alpha4 -&gt; satisfiable by drupal/simple_oauth[2.x-dev, 2.0.0-alpha4].


2.0-alpha4 requires Drupal 8.3. Leaving out the version specifier installs it as "1.0.0-beta5", which is however not listed anymore on the project page and therefore probably not really supported anymore.

8.3.0 will be released pretty soon, so if this is a new project, you could already switch to the recently released 8.3.0-rc1, there will likely only be very few changes until 8.3.0. See https://www.drupal.org/core/release-cycle-overview