I need help with fixing this error and I am using symfony 2.6
ServiceNotFoundException: The service "fos_user.security.login_manager" has a dependency on a non-existent service "security.context".
security.context
is deprecated since Symfony 2.6. You shouldn't have any problem because it's backward compatible, but you should check if you need to update your FOSUserBundle.
Run a composer update and check it.
More on the subject: http://symfony.com/blog/new-in-symfony-2-6-security-component-improvements
If this doesn't solve, try the steps in this Stackoverflow answer.