Hi Fixmobile ,
Unfortunately, that error has nothing to do with ATUM.
I've just checked the out-of-stock email functionality with only WooCommerce and ATUM active and the "Out of Stock Threshold per product" setting disabled, and it worked correctly on my end.
Please check that ATUM is updated, and if so, open a private ticket here. We'd need you to give us access to check your site settings.
Best Regards,
José Andrés