Error 1The type or namespace name 'DAL' could not be found

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data.SqlClient;
using System.Data;
namespace Management.DAL
{
    class DataAccessLayer
    {
        SqlConnection SqlConnection;

        public DataAccessLayer()
        {
            SqlConnection = new SqlConnection("server=MOHAMED; database=product_DB; Integrated Securit=true");
        }

       public void open()
        {
           if(SqlConnection.State != ConnectionState.Open)
           {
               SqlConnection.Open();
           }
        }
        public void Close()
       {
            if(SqlConnection.State==ConnectionState.Open)
            {
                SqlConnection.Close();
            }
       }

        public DataTable SelectData(string stored_procedure, SqlParameter[] param)
        {
            SqlCommand sqlcmd = new SqlCommand();
            sqlcmd.CommandType = CommandType.StoredProcedure;
            sqlcmd.CommandText = stored_procedure;

            if(param !=null)
            {
                for(int i = 0; i < param.length; i++)
                {
                    sqlcmd.Parameters.Add(param[i]);
                }
            }
            SqlDataAdapter da = new SqlDataAdapter(sqlcmd);
            DataTable dt = new DataTable();
            da.Fill(dt);
            return dt;
        }

        public void ExecuteCommand(string stored_procedure, SqlParameter[] param)
        {
            SqlCommand sqlcmd = new SqlCommand();
            sqlcmd.CommandType = CommandType.StoredProcedure;
            sqlcmd.CommandText = stored_procedure;

            if (param !=null)
            {
                sqlcmd.Parameters.AddRange(param);
            }

            sqlcmd.EndExecuteNonQuery();
        }
    }
}