Hi
here wrongly put content inside the ContentPlaceHolder in a master page.
so you have to put content as below.
<%@ Master Language="C#" AutoEventWireup="true" CodeBehind="UserMaster.master.cs" Inherits="HousingScocity.MasterPages.UserMaster.UserMaster" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<asp:ContentPlaceHolder ID="head" runat="server">
</asp:ContentPlaceHolder>
<style>
.vl {
border-left: 6px solid green;
height: 500px;
position: absolute;
left: 50%;
margin-left: -3px;
top: 0;
}
</style>
</head>
<body>
<form id="form1" runat="server">
<div>
<div>
<h1>WELCOME TO ONLIN E HOUING</h1>
<hr />
</div>
<div>
<asp:Button ID="homebtn" runat="server" Text="HOME" />
<asp:Button ID="Searchbtn" runat="server" Text="SEARCH" />
<asp:Button ID="RENTLISTbtn" runat="server" Text="RENTLIST" />
<asp:Button ID="SELLISTBTN" runat="server" Text="SELLISTBTN" />
<asp:Button ID="Contactbtn" runat="server" Text="Contact" />
</div>
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
</asp:ContentPlaceHolder>
<hr />
<div>
ALL RESERVE @2018
</div>
</div>
</form>
</body>
</html>
Click here for example