Cannot use object of type WP_Error as array (II)

Stack trace (using WooCommerce)

Error: Uncaught exception ‘Error’ with message ‘Cannot use object of type WP_Error as array’ in /wp-content/plugins/elastic-email-sender/api/ElasticEmailClient.php:58
in ElasticEmailClient\ApiClient::Request called at /wp-content/plugins/elastic-email-sender/api/ElasticEmailClient.php (1591)
in ElasticEmailClient\Email::Send called at /wp-content/plugins/elastic-email-sender/class/eemail.php (139)
in eemail::send called at /wp-content/plugins/elastic-email-sender/class/eemail.php (26)
in wp_mail called at /wp-content/plugins/woocommerce/includes/emails/class-wc-email.php (554)
in WC_Email::send called at /wp-content/plugins/woocommerce/includes/emails/class-wc-email-customer-completed-order.php (66)
in WC_Email_Customer_Completed_Order::trigger called at /wp-includes/class-wp-hook.php (286)
in WP_Hook::apply_filters called at /wp-includes/class-wp-hook.php (310)
in WP_Hook::do_action called at /wp-includes/plugin.php (515)
in do_action_ref_array called at /wp-content/plugins/woocommerce/includes/class-wc-emails.php (150)
in WC_Emails::send_transactional_email called at /wp-includes/class-wp-hook.php (286)
in WP_Hook::apply_filters called at /wp-includes/class-wp-hook.php (310)
in WP_Hook::do_action called at /wp-includes/plugin.php (453)
in do_action called at /wp-content/plugins/woocommerce/includes/class-wc-order.php (312)
in WC_Order::status_transition called at /wp-content/plugins/woocommerce/includes/class-wc-order.php (214)
in WC_Order::save called at /wp-content/plugins/woocommerce/includes/admin/meta-boxes/class-wc-meta-box-order-data.php (549)
in WC_Meta_Box_Order_Data::save called at /wp-includes/class-wp-hook.php (286)
in WP_Hook::apply_filters called at /wp-includes/class-wp-hook.php (310)
in WP_Hook::do_action called at /wp-includes/plugin.php (453)
in do_action called at /wp-content/plugins/woocommerce/includes/admin/class-wc-admin-meta-boxes.php (217)
in WC_Admin_Meta_Boxes::save_meta_boxes called at /wp-includes/class-wp-hook.php (288)
in WP_Hook::apply_filters called at /wp-includes/class-wp-hook.php (310)
in WP_Hook::do_action called at /wp-includes/plugin.php (453)
in do_action called at /wp-includes/post.php (3581)
in wp_insert_post called at /wp-includes/post.php (3654)
in wp_update_post called at /wp-admin/includes/post.php (377)
in edit_post called at /wp-admin/post.php (194)