Why it is not connecting with mysql database

Try this, but you need to change your connection string to your own. I have made many comments inline so I will keep this post to a minimum. If it helps, mark it as your answer. Any questions let me know. I've fixed a number of mistakes and commented some of them.

I've changed a lot of your code, and although I didn't write you a properly structured reusable robust class, as I didn't have time, but you should learn to utilise class development to your advantage, as you can make a lot of reusable code by doing so. Your problem is fixed for the most part.

Screenshot: https://prnt.sc/kv84hb[^]

using MySql.Data.MySqlClient; //Make sure this using directive is applied and reference your MySQL DLL to your project.
using System;
using System.Data;
using System.Windows.Forms;

namespace TestCSharpApp
{
    public partial class Form1 : System.Windows.Forms.Form
    {   //This is my connection string, and its working. I suggest you build yours the same way
        //Or consider using string builder to build your string
        private static string cString = "server=localhost;user id=root;password=root;persistsecurityinfo=True;database=visuals_db;port=3306";
        private static MySqlConnection Con = new MySqlConnection(cString);
        ////private static string server = "localhost";
        ////private static string database = "visuals_db";
        ////private static string uid = "root";
        ////private static string password = "root";

            //You don't need all these strings. Use the one string.

        public Form1()
        {
            InitializeComponent();//Do not use this method to store junk.
            //If you want to create new strings on load, use the form load event.
        }
        private void btnRegister_Click(object sender, EventArgs e)
        {
            string Fname = "John doe";
            string Uname = "JohnDoe1";
            string Email = "me@myemail.com";
            string Pass = "mypass123";

            if (Register(Fname, Uname, Email, Pass))
            {
                MessageBox.Show("User has been Created");
            }
            else
            {
                MessageBox.Show("User has not been Created");
            }
        }

        public bool Register(String Fname, String Uname, String Email, String Pass)
        {
            string execQuery = "INSERT INTO users (name,username,email,password) VALUES (@Fname,@Uname,@Email,@Pass)";
            //Remove your unnecessary ''''''' in values and don't encapsulate ; inside the querry.
            try //Do not nest try catch blocks, except, declare more than one catch exception
            {
                if (Con != null && Con.State == ConnectionState.Closed)
                    //Check connection is not null and open it if its closed.
                {
                    Con.Open();//Open con
                    using (MySqlCommand cmd = new MySqlCommand(execQuery, Con))
                    {//Use using blocks for disposable code
                        cmd.Parameters.AddWithValue("@Fname", Fname);
                        cmd.Parameters.AddWithValue("@Uname", Uname);
                        cmd.Parameters.AddWithValue("@Email", Email);
                        cmd.Parameters.AddWithValue("@Pass", Pass);
                        //Use parameterized command queries.
                        cmd.ExecuteNonQuery();
                        //Use add with value since add is depreciated
                    }
                    Con.Close();
                    //Remember to manually close the connection since we removed the unnecessary functions
                    return true;
                }
                return true;
            }
            catch (NullReferenceException nullRefEx)
            {
                MessageBox.Show(nullRefEx.Message);
                Con.Close(); //Check your connection state before attempting to open/close.
                return false;
            }
            catch (MySqlException sqlErr)
            { // handle your second error here
                return false;
            }
            catch (Exception ex)
            {//Then catch all errors you might miss?
                return false;
            }
            finally
            {//https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/try-finally
                //Finish your statements if any cleaning up needed etc...
            }
        }
    }
}