This error message means that there is an issue with your PHP version. Please make sure to use at least PHP7.0 or higher. You can verify the PHP version of your WooCommerce installation under WooCommerce > Status > System Status > Server environment.
Please note: In some cases this error is triggered during the execution of the cron job. The PHP version used in a server cron job can be different from the PHP version used by your webserver. Please make sure your server cron job uses at least PHP7.0 or higher or use the alternative cron job command that starts with "wget" instead of the default command.