Unable to create an admin menu link
It looks like you are using an older version of Drupal core. Upgrade to one of the betas or the latest 8.0.x from Git and it should work fine.

In the latest versions the filenames have been changed, as reported in YAML files for menu links, contextual links, local tasks, and local actions have been renamed. 8.0.0-alpha14 is the first tagged release with the filename change.