application.html.erbに以下が書かれているかをチェック
application.html.erb
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
いか、修正後。
application.html.erb
<!DOCTYPE html>
<html>
<head>
<title>NarouOsusume</title>
<%= csrf_meta_tags %>
<%= csp_meta_tag %>
<%= stylesheet_link_tag 'application', media: 'all', 'data-turbolinks-track': 'reload' %>
<%= javascript_include_tag 'application', 'data-turbolinks-track': 'reload' %>
<!-- ここ！！！-->
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
<!-- ここ！！！-->
</head>
<body>
<%= yield %>
</body>
</html>