How to create permission to allow access to an admin form?
I think you can omit restrict access and remove the _ from your permission name. 

Anyway, you have a typo in your permission key. It should be _permission instead of _permissions. See: https://www.drupal.org/docs/8/api/routing-system/introductory-drupal-8-routes-and-controllers-example

Make that change and clear the cache.