How to fix 'entityvalidationerrors' in C#

I tried to make the application and connect it to the SQL Server. but I have a problem with the following line of code.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.IO;

namespace CBIR
{
    public partial class training : Form
    {
        string fileName;
        public training()
        {
            InitializeComponent();
        }

        private void Pilihgbr_Click(object sender, EventArgs e)
        {
            using (OpenFileDialog dialog = new OpenFileDialog() { Filter = "JPG Files(*.jpg)|*.jpg|PNG Files(*.png)|*.png|All Files(*.*)|*.*", ValidateNames = true, Multiselect = false })
            {
                if (dialog.ShowDialog() == DialogResult.OK)
                {
                    fileName = dialog.FileName;
                    pictureBox1.Image = Image.FromFile(fileName);
                }
            }
        }

        byte[] ConvertImage (Image img)
        {
            using (MemoryStream ms = new MemoryStream())
            {
                img.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
                return ms.ToArray();
            }
        }

        private async void Mulaitraining_Click(object sender, EventArgs e)
        {
            using (cbirEntities db = new cbirEntities())
            {
                tabel_cbir pic = new tabel_cbir() { Nama_Gambar = fileName, Gambar = ConvertImage (pictureBox1.Image)};
                db.tabel_cbir.Add(pic);
                await db.SaveChangesAsync();
                MessageBox.Show("Tersimpan", "Message", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
        }
        private void balik_Click(object sender, EventArgs e)
        {
            menuutama m1 = new menuutama();
            m1.Show();
            this.Close();
        }
    }
}

and an error occurred while running the program on the line.

await db.SaveChangesAsync();

Any help will be appreciated. Thanks.