// my BooksController.cs Code
      public ActionResult Index()
      {
             var books = db.Books.Include(h => h.BorrowHistories)
             .Select(b => new BookViewModel
             {
                  BookId = b.BookId,
                  Author = b.Author,
                  Publisher = b.Publisher,
                  SerialNumber = b.SerialNumber,
                  Title = b.Title,
                  IsAvailable = !b.BorrowHistories.Any(h => h.ReturnDate == null)  //Error is coming in this line
              }).ToList();
        return View(books);
       }

// my Book.cs
        using System;
        using System.Collections.Generic;
        using System.Linq;
        using System.Web;
        using System.ComponentModel.DataAnnotations;
        using System.Collections;

        namespace LibraryManagmentSystem.Models
        {
         public class Book
         {
           public int BookId { get; set; }
           [Required]
           public string Title { get; set; }
           [Required]
           [Display(Name = "Serial Number")]
           public string SerialNumber { get; set; }
           public string Author { get; set; }
           public string Publisher { get; set; }
           public ICollection BorrowHistories { get; set; }

          }
         }

public ICollection<YourSubClass> BorrowHistories { get; set; }

public ICollection BorrowHistories { get; set; }