Try this,
This will create your plugin menu appear at the wordpress admin menu levels.
//Call hook admin_menu to add plugin menu in WP admin section
add_action('admin_menu', 'register_my_menu_page');
//Main function to populate plugin menus
function register_my_menu_page() {
add_menu_page('Employee Register', 'Attendance', 'manage_options', 'at-main-menu', 'AtShowReport');
add_submenu_page("at-main-menu", "Emp-timeoff-mast", "Report", 1, "at-main-menu", "fucntion_name");
add_submenu_page("at-main-menu", "Manage Employees", "Manage Employees", 1, "manage-emp", "fucntion_name");
add_submenu_page("at-main-menu", "Register Employee off time", "Register Time off", 1, "emp-reg-off-time", "fucntion_name");
add_submenu_page("at-main-menu", "Employee status", "Status", 1, "emp-status", "fucntion_name");
add_submenu_page("at-main-menu", "Holidays", "Holidays", 1, "emp-holiday", "fucntion_name");
}
function_name is the name of the fucntion that get executed when the menu is clicked
Refer following link to get better understanding of these functions.
https://codex.wordpress.org/Plugin_API/Action_Reference/admin_menu
https://codex.wordpress.org/Function_Reference/add_menu_page
https://codex.wordpress.org/Function_Reference/add_submenu_page