GNUPAY - KG Inicis 에 키 파일이 업로드되지 않습니다.

에러 메시지는 아래와 같은데, 어떻게 수정하면 될까요?

Fatal error: Uncaught Error: Call to undefined function WP_Filesystem() in /fashionmakers1/www/wp-content/plugins/gnupay-inicis/classes/inicis_card_gateway.class.php:1092 Stack trace: #0 /fashionmakers1/www/wp-content/plugins/gnupay-inicis/classes/inicis_card_gateway.class.php(883): inicis_card_gateway->key_file_upload(Array) #1 /fashionmakers1/www/wp-content/plugins/woocommerce/includes/abstracts/abstract-wc-settings-api.php(152): inicis_card_gateway->validate_inicis_key_field('de_inicis_mid', '') #2 /fashionmakers1/www/wp-content/plugins/woocommerce/includes/abstracts/abstract-wc-settings-api.php(212): WC_Settings_API->get_field_value('de_inicis_mid', Array, Array) #3 /fashionmakers1/www/wp-content/plugins/gnupay-inicis/classes/inicis_card_gateway.class.php(118): WC_Settings_API->process_admin_options() #4 /fashionmakers1/www/wp-includes/class-wp-hook.php(287): inicis_card_gateway->process_admin_options('') #5 /fashionmakers1/www/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters('', Array) #6 /fashionmakers1/ww in /fashionmakers1/www/wp-content/plugins/gnupay-inicis/classes/inicis_card_gateway.class.php on line 1092