Assign Layout page inside _ViewStart.cshtml in views folder.
@{
Layout = "~/Views/Shared/_Layout.cshtml";
}
If you missed the @RenderBody() inside _Layout.cshtml(Layout page) you will get above error. So you must include it.
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width" />
<title>@ViewBag.Title</title>
@Styles.Render("~/Content/css")
@Scripts.Render("~/bundles/modernizr")
</head>
<body>
@RenderBody()
@Scripts.Render("~/bundles/jquery")
@RenderSection("scripts", required: false)
</body>
</html>