Hi lydia ,
thanks for your post.
That error arises because the plugin can't find or open the file "/atum-stock-manager-for-woocommerce/vendor/composer/autoload_real.php".
This file is part of ATUM and it's included in the installed files when you download/install our plugin.
Please, access the server and check if the file is there. If so, it must be that for some reason the file does not have the correct permissions (if you do not know how to manage them, please, contact your hosting service provider), else, if it is not there, please try removing/reinstalling ATUM.
Hope this helps,