ASP.NET

How to fix error "the type or namespace name 'CustomerBAL' could not be found (are you missing a using directive or an assembly reference) 

Name project MvcApplication1) on ASP.Net Visual Studio 2012 
Could anyone help me ? Thank's in advance ...

public class Book
    {
        [Key]
        public int Id { get; set; }
        [Required]
        [MaxLength(30)]
        public string Title { get; set; }
        public string Authers { get; set; }
 
        [Column("Year")]
        [Display(Name = "Publish Year")]
        public string publishYear { get; set; }
 
        [Column("Price")]
        [Display(Name = "Price")]
        public decimal BasePrice { get; set; }
    }

public interface IBookRepository : IDisposable
        {
            IEnumerable<Book> GetBooks();
            Book GetBookByID(int bookId);
            void InsertBook(Book book);
            void DeleteBook(int bookID);
            void UpdateBook(Book book);
            void Save();
    }

public class BookRepository : IBookRepository
    {
        private BookContext _context;
 
        public BookRepository(BookContext bookContext)
        {
            this._context = bookContext;
        }
 
        public IEnumerable<book> GetBooks()
        {
            return _context.Books.ToList();
        }
 
        public Book GetBookByID(int id)
        {
            return _context.Books.Find(id);
        }
 
        public void InsertBook(Book book)
        {
            _context.Books.Add(book);
        }
 
        public void DeleteBook(int bookID)
        {
            Book book = _context.Books.Find(bookID);
            _context.Books.Remove(book);
        }
 
        public void UpdateBook(Book book)
        {
            _context.Entry(book).State = EntityState.Modified;
        }
 
        public void Save()
        {
            _context.SaveChanges();
        }
 
        private bool disposed = false;
 
        protected virtual void Dispose(bool disposing)
        {
            if (!this.disposed)
            {
                if (disposing)
                {
                    _context.Dispose();
                }
            }
            this.disposed = true;
        }
 
        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }
    }

Protected Sub txtEmpID_TextChanged(sender As Object, e As EventArgs) Handles txtEmpID.TextChanged
    If Not String.IsNullOrEmpty(txtEmpID.Text) Then
        Dim Conn As SqlConnection
        'Read in connection String
        Conn = New SqlConnection(ConfigurationManager.ConnectionStrings("constr").ConnectionString)
        Conn.Open()
        Dim cmd As New SqlCommand("select * from Employees e inner join dateDetails d on e.employeeID = d.EmployeeID where EmpID=@empID and Year(d.enterDate) >= 2016  and Year(d.enterDate)<= 2017", Conn)
        cmd.Parameters.AddWithValue("@empID", txtEmpID.Text)
        Dim dr As SqlDataReader = cmd.ExecuteReader()
        If dr.HasRows Then
            dr.Read()
            checkusername.Visible = True
            'imgstatus.ImageUrl = "images/NotAvailable.jpg"
            lblStatus.Text = "Please verify your information for accuracy. Then complete rest of the form."
            lblStatus.ForeColor = System.Drawing.Color.Red
            System.Threading.Thread.Sleep(100)
            txteName.Text = dr("employeeName").ToString()
            txttitle.Text = dr("empTitle").ToString()
            txtemail.Text = dr("email").ToString()
            txtEmpID.Text = dr("empID").ToString()
        Else
            dr.Read()
            checkusername.Visible = True
            'imgstatus.ImageUrl = "images/Icon_Available.gif"
            lblStatus.Text = "Proceed"
            lblStatus.ForeColor = System.Drawing.Color.Red
            System.Threading.Thread.Sleep(100)
            txteName.Text = dr("employeeName").ToString()
            txttitle.Text = dr("empTitle").ToString()
            txtemail.Text = dr("email").ToString()
            txtEmpID.Text = dr("empID").ToString()
        End If
    Else
        checkusername.Visible = False
    End If
End Sub

var response = await client.GetAsync("ServerConfig");

private WebApiClientOptions options = new WebApiClientOptions("http://localhost:54990/api", "ServerConfig");

    protected void Page_Load(object sender, EventArgs e)
    {
        List<ServerConfig> list = null;

        list = this.Index().Result;
      
    }

    public async Task<List<ServerConfig>> Index()
    {
        List<ServerConfig> list = null;

        using (WebApiClient<ServerConfig> client = new WebApiClient<ServerConfig>(options))
        {
            var liste = await client.GetManyAsync();

        }

        return list;
    }