1>------ Сборка начата: проект: Forms, Конфигурация: Release Any CPU
------ 1>C:UsersСеняDocumentsVisual Studio 2017ProjectsSeniastr.StudentsSeniastr.StudentsForm2.cs(26,22,26,33): error CS1061: 'Student" не содержит определения для "HaveStepend" и не удалось найти метод расширения "HaveStepend", принимающий тип "Student" в качестве первого аргумента (возможно, пропущена директива using или ссылка на сборку).
========== Сборка: успешно: 0, с ошибками: 1, без изменений: 0, пропущено: 0 ==========

public Form2(User user)
        {
            if (user != null)
            {
                this.student = user;
                InitializeComponent();
                this.name_tb.Text = user.Student.Fullname;
                this.cours_tb.Text = user.Student.Cours.ToString();
                this.reputation_l.Text = $"Репутация: {user.Student.Reputation}";
                if (user.Student.HaveStepend == true) // Отсюда вызов
                {
                    this.stepend.Text = $"Степендия: есть";
                }
                else
                {
                    this.stepend.Text = $"Степендия: нету";
                }
                this.Text = $"{user.Student.Fullname}, {user.Student.Cours} курс.";
            }
            else
            {
                MessageDialog message = new MessageDialog("Ошибка!", "Ошибка, для исправления обратитесь к разработчику!");
            }
        }

public partial class Form1 : Form
        {
...
private void OK_b_Click(object sender, EventArgs e)
        {

            User user = new User(new Username(username_tb.Text),new Password(password_tb.Text));
            if (CheckInUsers(user, out user))
            {
                Form2 form = new Form2(user);
                Task task = new Task(() => { this.Dispose(); });
                Task task2 = new Task(() => { this.Visible = false; form.ShowDialog();  });
                task2.Start();
                task.Start();
                return;
            }
            else {
                MessageDialog message = new MessageDialog("Ошибка", "Ошибка, такого пользователя не обнаружено.");
                message.Show();
            }
        }
...
    }

public class Student
    {
        private int cours;
        private string fullname;
        private float reputation;
        private User user;
        private Post post;
        private bool havestepend = true;

        public bool HaveStepend { get => havestepend; set => havestepend = value; }
        public float Reputation { get { return reputation; }}
        public string Fullname { get { return fullname; }set { fullname = value; } }
        public int Cours { get { return cours; } set { cours = value; } }
        public User User { get { return user; } set { user = value; } }
        public Post Post { get => post;}

        public Student()
        {
            reputation = 0f;
            this.fullname = "";
            cours = 0;
            this.havestepend = false;
        }
        public Student(string fullname)
        {
            reputation = 0f;
            this.fullname = fullname;
            cours = 0;
            this.havestepend = false;
        }
        public Student(string fullname, int cours)
        {
            reputation = 0f;
            this.fullname = fullname;
            this.cours = cours;
            this.havestepend = false;
        }
        public Student(string fullname, int cours, float reputation)
        {
            this.reputation = reputation;
            this.fullname = fullname;
            this.cours = cours;
            this.havestepend = false;
        }
        public Student(string fullname, int cours, float reputation, User user)
        {
            this.reputation = reputation;
            this.fullname = fullname;
            this.cours = cours;
            this.user = user;
            this.havestepend = false;
        }
        public Student(string fullname, int cours, float reputation, User user, Post post)
        {
            this.reputation = reputation;
            this.fullname = fullname;
            this.cours = cours;
            this.user = user;
            this.post = post;
            this.havestepend = false;
        }

public class User
    {
        private Username username;
        private Password password;
        private Student student;
        public static List<User> list = new List<User>();

        public User(Username username, Password password, Student student)
        {
            this.username = username;
            this.password = password;
            this.student = student;
            list.Add(this);
        }
        public User(Username username, Password password)
        {
            this.username = username;
            this.password = password;
        }


        public Password Password { get => password; set => password = value; }
         public Username Username { get => username; set => Username = value; }
         public Student Student { get => student; }