I have changed the "_Layout.cshtml" like below and it works. .Net Core 3.1 Blazor Server module Identity issue.
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no" />
<title>@ViewBag.Title</title>
<link href="~/css/bootstrap/bootstrap.min.css" rel="stylesheet" />
<link href="~/css/app.css" rel="stylesheet" />
@RenderSection("Scripts", required: false)
</head>
<body>
<div class="main">
<div class="content px-4">
@RenderBody()
</div>
</div>
</body>
</html>
@section Scripts {
}