First the follow will not render I have replaced the code with text and it works so I know rails is finding the file just not rendering the form second If you look at the _follow_form where I'm calling @post.user_id I' getting a error undefined method user_id for nil nil class

_follow.rb

<%= form_for(current_user.active_relationships.build, remote: true) do |f| %>
  <div><%= hidden_field_tag :followed_id, user.id %></div>
  <%= f.submit 'Follow', class: 'btn btn-primary'%>
<% end %>

follow_form.rb

<div id="follow_form">
  <% if current_user.following?(@post.user_id) %>
   <%= render 'follow' %>
  <% else %>
   <%= render 'unfollow' %>
  <% end %>
</div>

index.rb

<div class="container">
  <% @posts.each do |post| %>
    <div class="col-sm-6 col-md-4">
      <div class="thumbnail thumb-box">
        <%= link_to image_tag post.image.url(:medium) %>
        <h5 style = "text-align:center;"><%= link_to post.user_name, post_path(post) %> | <%= post.title %></h5>
        <% if current_user %>
        <div class="btn btn-default" >
          <%= link_to image_tag("glyphicons-402-skateboard.png"),like_post_path(post), method: :put, data: { remote: true } %>Like
          <span id="post_<%= post.id %>"><%= post.get_upvotes.size %></span>
        </div>
        <%= link_to "Edit", edit_post_path(post) %> | <%= link_to "Delete", "#"%>
        <%= render 'follow_form'if current_user != nil %>
        <% end %>
      </div>
    </div>
  <% end %>
</div>

post_controller.rb

class PostsController < ApplicationController
  def new
    @post = current_user.posts.build
  end

  def show
    @post = Post.find(params[:id])
    @post_comment  = @post.comments.build
    @post_comments = @post.comments
  end

  def index
    @posts  = Post.order("created_at DESC")
  end

  def edit
    @post = Post.find(params[:id])
  end

  def upvote
    @post = Post.find(params[:id])
    @post.upvote_by current_user
    respond_to do |format|
      format.js   { render 'vote' }
    end
  end

  def create
    @post = current_user.posts.build(post_params)
    @post.user.id = current_user.id
    if @post.save
      redirect_to posts_path
    else
      render 'new'
    end
  end


  def post_params
    params.require(:post).permit(:user_name, :title, :body, :image, :video )
  end
end

You do not have @post in the index action, you have @posts. So you should pass post in this line as locals like <%= render 'follow_form', post: post %> and here change to <% if current_user.following?(post.user_id) %>. For show and edit actions you should also pass variable to form when render like <%= render 'follow_form', post: @post %>.