Cannot open the DB error

int userId = 0;

string roles = string.Empty;

string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;

using (SqlConnection con = new SqlConnection(constr))
{
    using (SqlCommand cmd = new SqlCommand("Validate_User"))
    {
        cmd.CommandType = CommandType.StoredProcedure;

        cmd.Parameters.AddWithValue("@Username", Login1.UserName);
        cmd.Parameters.AddWithValue("@Password", Login1.Password);
        cmd.Connection = con;

        con.Open();

in the web.config, the connection string is defined as

<add name="constr" 
     connectionString="data source=.;initial catalog=LoginDB;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework" 
     providerName="System.Data.SqlClient" />

but when I run it, I get an error:

System.Data.SqlClient.SqlException: Cannot open database "LoginDB" requested by the login. The login failed.
Login failed for user 'NT AUTHORITY\SYSTEM'.

Any solutions?