Error: A configuration file cannot be created for the requested Configuration object

Hi Guys

Can an expert help me solving updating <appsettings><add key='name" value='value'></appsettings> in the Web.Config?

The Project is a Web-Form ASP.NET C# Using Framework 4.7.2

Error Thrown "A configuration file cannot be created for the requested Configuration object."

<h2>
        Updating Web.config Settings
    </h2>
    <p>
        <asp:TextBox ID="asetKeyName" runat="server"></asp:TextBox>
        <asp:TextBox ID="asetKeyValue" runat="server"></asp:TextBox>
        <asp:Button ID="btnUpdate" Text="Update AppSettings" OnClick="btnUpdate_Click" runat="server" />

    </p>
    <p>
        <asp:Literal ID="lit_Content" runat="server"></asp:Literal>
    </p>

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            //- Default name of the Key
            this.asetKeyName.Text = "max-attendance";
        }
    }
    protected void btnUpdate_Click(object sender, EventArgs e)
    {
        try
        {
            var keyName = this.asetKeyName.Text.Trim();
            var keyValue = this.asetKeyValue.Text.Trim();
            var config = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration("~/web.config");
            config.AppSettings.Settings[keyName].Value = keyValue;
            config.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection("web.config");
            this.lit_Content.Text = "Successfully Updated.";
        }
        catch (Exception exm)
        {
            this.lit_Content.Text = exm.Message.ToString();
            Console.WriteLine(exm.Message.ToString());
        }
    }
}

<?xml version="1.0"?>
<!--
  For more information on how to configure your ASP.NET application, please visit
  http://go.microsoft.com/fwlink/?LinkId=169433
  -->
<configuration>
  <appSettings>
    <add key="max-attendance" value="10"/>
  </appSettings>
  <connectionStrings>
    <add name="ApplicationServices" connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|\aspnetdb.mdf;User Instance=true" providerName="System.Data.SqlClient"/>
  </connectionStrings>
  <system.web>
    <compilation debug="true" targetFramework="4.0"/>
    <authentication mode="Forms">
      <forms loginUrl="~/Account/Login.aspx" timeout="2880"/>
    </authentication>
    <membership>
      <providers>
        <clear/>
        <add name="AspNetSqlMembershipProvider" type="System.Web.Security.SqlMembershipProvider" connectionStringName="ApplicationServices" enablePasswordRetrieval="false" enablePasswordReset="true" requiresQuestionAndAnswer="false" requiresUniqueEmail="false" maxInvalidPasswordAttempts="5" minRequiredPasswordLength="6" minRequiredNonalphanumericCharacters="0" passwordAttemptWindow="10" applicationName="/"/>
      </providers>
    </membership>
    <profile>
      <providers>
        <clear/>
        <add name="AspNetSqlProfileProvider" type="System.Web.Profile.SqlProfileProvider" connectionStringName="ApplicationServices" applicationName="/"/>
      </providers>
    </profile>
    <roleManager enabled="false">
      <providers>
        <clear/>
        <add name="AspNetSqlRoleProvider" type="System.Web.Security.SqlRoleProvider" connectionStringName="ApplicationServices" applicationName="/"/>
        <add name="AspNetWindowsTokenRoleProvider" type="System.Web.Security.WindowsTokenRoleProvider" applicationName="/"/>
      </providers>
    </roleManager>
  </system.web>
  <system.webServer>
    <modules runAllManagedModulesForAllRequests="true"/>
  </system.webServer>
</configuration>

Thank you in advanced for your help.