It seems that your function wstock_variation not exists or is created in PHP file not include in your plugin…
If you are using OOP, for example your code need to be like:
add_submenu_page( 'Elites-StockManager', 'Smart Stock Manager', 'export', 'administrator', 'wstock_variation', array($this, 'wstock_variation') ); // remove number 8, method accept only 6 params
$this is method is not static, in case of static method you need to change by class name.