posts_controller.rb

...
  def new
    @branch = params[:branch]
    @categories = Category.where(branch: @branch)
    @post = Post.new
  end

  def create
    @post = Post.new(post_params)
    if @post.save 
      redirect_to post_path(@post) 
    else
      redirect_to root_path
    end
  end
...