Use current class method inside add_submenu_page()

Question

You’re getting that error because the 5th parameter for add_submenu_page() should be a string which is the menu slug and yet you supplied a callback ([$this,'sports_info_page']). So be sure to use a valid slug like so and the error will be gone:

add_submenu_page(
    'sports_info',             // parent slug
    'Sports Information',      // page title
    'Sports Information',      // menu title
    'manage_options',          // capability
    'your-menu-slug',          // menu slug
    [$this,'sports_info_page'] // callback
);