create submenu page – error function not found or invalid function name

I have created a plugin with a top level page. I created a sub menu underneath that. When I run the plugin on my site everything works. But when I test it on a separate site I get an error and I’m not sure why.

This is the the code that is in question and the error that is getting thrown. I can’t for the life of me figure out what the error is from.

function custom_plugin_add_menu_page(){

     add_menu_page( 'My Custom Plugin', 
                         'My Custom Plugin', 
                         'manage_options', 
                         'top_level_parent_page', 
                         'top_level_parent_page',
                         plugins_url( '/my-custom-plugin/includes/images/menu-icon.png' )
                         );                           

     add_submenu_page( 
        'top_level_parent_page', 
        'Custom Submenu Page', 
        'Custom Submenu Page',
        'manage_options', 
        'custom_plugin_submenu_page', 
        'custom_plugin_submenu_page_callback' 
    );                  
}
add_action('admin_menu', 'custom_plugin_add_menu_page');

and the error:

Warning: call_user_func_array() expects parameter 1 to be a valid
callback, function ‘custom_plugin_submenu_page_callback’ not found or
invalid function name in
D:xampphtdocslocaltestsitewp-includesplugin.php on line 429