Trying to check if user exists in my database

I am trying to check if user exists in my SQL Server database, but if user exists, or not, I always get the same response (redirection to ~/korisnik/Korisnikkp. Can someone help me? Problem is compiler doesn't get username value with this @0. Do you know how to fix that? If i hardcode user with examle 'nicolas55' it works. Thanks

    if (IsPost)
{
    var Username = "";
    var Password = "";
    Username = Request.Form["Username"];
    Password = Request.Form["Password"];
    if (Username != "" && Password != "")
    {
        var db = Database.Open("Useri");
        var command = @"SELECT username FROM Users WHERE username = @Username"; //when I put here 'nikola55' it works
        var result = db.QueryValue(command, Username);


        if (result != null)
        {
            Response.Redirect("~/korisnik/Korisnikkp");
        }
        else
        {
            <script type="text/javascript">
                alert("No user in database");
            </script>
        }

Form code:

<form method="post">
                        <!-- to error: add class "has-danger" -->

                        <label for="exampleInputEmail1">username</label>
                        <input type="text" value="@Username">
                        <label for="exampleInputPassword1">Password</label>
                        <a href="#" style="float:right;font-size:12px;">Forgot password?</a>
                        <input type="password" value="@Password">

                        <button type="submit" class="btn btn-primary btn-block">Sign in</button>

                        <div class="sign-up">
                            Don't have an account? <a href="/korisnik">Create One</a>
                        </div>
                    </form>