Can’t enable multiple warehouse

Hello,
I installed and activated the plugin, but it does not allow me to check the box to enable multiple warehouses under the warehouse tab in WooCommerce setting. I am using the latest versions of Woo and WordPress.
No error message is displayed, but if I look under stock status in a specific product, it says “Warning: sizeof(): Parameter must be an array or an object that implements Countable in /wp-content/plugins/warehouse-popups-woocommerce/admin/class-warehouse-popups-woocommerce-admin.php on line 718”

What can I do to make it work?