mysql_affected_rows was deprecated in PHP 5.5.0, and it was removed in PHP 7.0.0. Instead, the MySQLi or PDO_MySQL extension should be used. [Refer: https://www.php.net/manual/en/functi...cted-rows.php]

On your web server, if you have PHP Extensions mysqli ticked simply change
mysql_affected_rows($db->link)), 'back_in_stock_notifications'); to
mysqli_affected_rows($db->link)), 'back_in_stock_notifications');
at approx line 273 in \admin\includes\functions\back_in_stock_notifications_functions.php


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environment Dev: Zen Cart 156c; Apache 2.4.33; PHP 7.3.1; MySQL 5.7.21; Windows 10
environment Test: Zen Cart 156a; Apache 2.4.29; PHP 7.2.4; MySQL 5.7.24; Linux 4.15.0

packages / add-ons:
ckeditor ; clone template 1.2.0; payment module fee; Direct bank Deposit V1.5.revised; zencart155 securepayxml (modified); ozpost v4.2.7; ceon_back_in_stock_notifications.9 (modified); export_shipping_information_V1.3.5; edit_orders-4.3.5; master password 4; admin log in as customer; ZCA bootstrap template-for-156-v2.0.0c; image checker 2.0; sales report 3.2.2; email archive manager 1-8e; zenNoncaptcha v0.5