Hi, since WooCommerce 3.0 we have deprecated the reduce_order_stock function in the Order class and replaced it with a global function called wc_reduce_stock_levels.
All that message means is you still have a theme or plugin that is utilising the old function that will need to be updated to use the new function. I can unfortunately not tell you which plugin it might be so you will have to troubleshoot on your own. I would start by checking if there are updates available for any plugins you have installed and then update, and also check your theme for updates and update that.
Remember that you need to make sufficient backups for updating your theme or plugin in case something goes wrong you have a way to roll back the changes.
