NameError in PostsController # index | uninitialized constant PostsController :: Post

Make sure that the Post class is correctly set in the file app/models/post.rb file of your application and if it is implemented similar to:

class Post
  # código
end

Accidents do happen. One piece of advice I give you is to version your application code with a version control system such as Git ( link ). In this way, any accidental change or loss of code can be easily reversed.

You did not import your Post model inside the Controller, so it can not identify the class.

class PostController < ApplicationController
  require 'models/post'

  def index
    @posts = Post.All
  end
end