"undefined method `errors' for nil:NilClass" when calling on errors method

You need to define @post in your new action too.

def new
  @post = Post.new
end

You're getting the NilClass error because @post has no value (it's nil) when you first load the form on the new action.

When you do the render :new in your create action there is no problem because it's using the @post you've defined at the top of create.