WordPress: add_submenu_page() creating error

Solution:

Try the code below in your functions.php file. It should work correctly.

add_menu_page(
    'Menu Page Title', // page title
    'Menu Menu Text', // menu link text
    'manage_options', // capability to access the page
    'menu_slug', // page URL slug
    'menu_callback_function', // callback function to display the content on options page
    'dashicons-format-status', // menu icon
    2 // priority
);

add_submenu_page(
    'menu_slug', // page URL slug
    'Sub Menu Title', // page title
    'Sub Menu Text', // menu link text
    'manage_options', // capability to access the page
    'submenu_slug', // page URL slug
    'submenu_callback_function', // callback function to display the content on options page
    1 // priority
);

function menu_callback_function(){
    echo "This is menu page...";
}

function submenu_callback_function(){
    echo "This is submenu page...";
}