Error trying to store a fingerprint in .asp

I am implementing a fingerprint reader in .asp and this is done in an event where every time you read a fingerprint you should store it in B64 fingerprint but send me an error I leave the code I hope you can help me greetings

Code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using ZKFPEngXControl;
using System.Threading;
using System.IO.Ports;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {      
    }
    ZKFPEngX a = new ZKFPEngX();
    string huellaB64 = "";          

    protected void Button1_Click(object sender, EventArgs e)
    {
        a.SensorIndex = 0;
        a.FPEngineVersion = "9";
        a.InitEngine();

        if (a.Active)
        {
            Label1.Text = "Sensor activo";
        }
        else
        {
            Label1.Text = "Sensor desconctado";
        }
        a.OnCapture += A_OnCapture;        
    }

    private void  A_OnCapture(bool ActionResult, object ATemplate)
    {     
        huellaB64 = a.GetTemplateAsString();
        Label1.Text = huellaB64;
        throw new NotImplementedException();
    }
}