ruby on rails - can't add a new post

Processing by PostsController#create as HTML Parameters: {"utf8"=>"✓", "authenticity_token"=>"DXrr78QugzRU4TQ+8StPOR79VE/Fd8DFKzBSJyd1LKFrb3hhcXAjOyTj1eVxb3Tz1QbLGHdXC3R5oboDFSTI1w==", "post"=>{"title"=>"Blog post", "body"=>"does this work?", "category_id"=>"1"}, "commit"=>"Add Post"} (0.0ms) BEGIN (1.0ms) ROLLBACK Rendering posts/new.html.erb within layouts/application