_navigation.html.eex

... <!-- Code removed for readability -->

<div class="top-bar-right">
    <ul class="menu">
      <%= if @conn.assigns[:user] do %>
        <li class="menu-text">
          Welcome, <%= @conn.assigns.user.first_name %>!
        </li>
        <li>
          <%= link "Videos", to: video_path(@conn, :index) %>  <!-- Update this line -->
        </li>
        <li>
          <%= link "Add video", to: video_path(@conn, :new) %>  <!-- Update this line -->
        </li>
        <li>
          <%= link "Sign out", to: auth_path(@conn, :signout), class: "button alert" %>
        </li>
      <% else %>
        <li>
          <%= link "Videos", to: video_path(@conn, :index) %>  <!-- Update this line -->
        </li>
        <li>
          <%= link "Sign in with Google", to: auth_path(@conn, :request, "google", scope: "email profile"), class: "button" %>
        </li>
      <% end %>
    </ul>
  </div>
  
  ... <!-- Code removed for readability -->

<%= link "Sign out", to: auth_path(@conn, :signout), class: "button alert" %>

Should be:

<%= link "Sign out", to: auth_path(@conn, :delete), class: "button alert" %>