Settings options not showing up on Sub Menu page in WordPress plugin

If you check the arguments for add_menu_page, you’ll see that the fifth argument is your callback function. And, right now, there’s nothing in the mj_admin function.

What you need to do is output your form in that function.

<?php
function mj_admin(){
?>
    <h2>My Settings Page</h2>
    <form action="options.php" method="post">
<?php
    settings_fields( 'mj-admin-settings' );
    do_settings_sections('mj-admin-settings');
    submit_button();
?>
    </form>
<?php
}