register_activation_hook() not working as expected

Question

This is the main plugin file. It activates but nothing happens. The menu is not created.

/*
 *  Activate plugin
 *    create admin menu
 *    create more stuff
 */
register_activation_hook(__FILE__, 'markmast_activate');

function markmast_activate() {
  add_action('admin_menu', 'markmast_create_admin_menu');
        
  function markmast_create_admin_menu() {
    add_menu_page( 'Marketing Master', 'Marketing Master Settings', 'manage_options', 'custom');
  }
}

Answer 1

WP needs all the required arguments - you skipped the one that tells it either a PHP file to use for output, or a function to run.

From the Developer Site:

/**
 * Register a custom menu page.
 */
function wpdocs_register_my_custom_menu_page(){
    add_menu_page( 
        __( 'Custom Menu Title', 'textdomain' ),
        'custom menu',
        'manage_options',
        'custompage',
        'my_custom_menu_page',
        plugins_url( 'myplugin/images/icon.png' ),
        6
    ); 
}
add_action( 'admin_menu', 'wpdocs_register_my_custom_menu_page' );
 
/**
 * Display a custom menu page
 */
function my_custom_menu_page(){
    esc_html_e( 'Admin Page Test', 'textdomain' );  
}

The 5th argument in add_menu_page() is the name of a custom function, which is then defined below.