Only top-level controls can have an owner.

using MetroFramework.Forms;
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 BillingSystem.Presentation;

namespace BillingSystem.Presentation
{
    public partial class frmMainMenu : MetroForm
    {
        public frmMainMenu()
        {
            InitializeComponent();
        }
        public void LoadForm(MetroForm frm)
        {
            frm.MdiParent = this;
            frm.WindowState = FormWindowState.Maximized;
            frm.BringToFront();
            frm.Show();                               
        }
        private void frmMainMenu_Load(object sender, EventArgs e)
        {
           
        }
    
        private void metroButton2_Click(object sender, EventArgs e)
        {
            try
            {
                LoadForm(new frmAddInvoice());
                this.WindowState = FormWindowState.Maximized;
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }        
    }
}