Declare HPOS compatibility

Please declare that you are compatible, so WooCommerce knows that there’s no incompatible plugins. Currently it thinks you’re incompatible and won’t let HPOS be enabled.

https://github.com/woocommerce/woocommerce/wiki/High-Performance-Order-Storage-Upgrade-Recipe-Book#declaring-extension-incompatibility

It only requires adding a couple of lines to the plugin.

/wp-admin/plugins.php?plugin_status=incompatible_with_feature&feature_id=custom_order_tables will show you the issue.