Fatal error: Uncaught Error during checkout

I have re-installed the plugin, de-activated plugins and changed themes with no success.

Error when attempting to check out through NMI Gateway for WooCommerce:

Fatal error: Uncaught Error: Call to undefined function wcs_is_subscription()
in /home/customer/www/warhorseconcepts.com/public_html/wp-content/plugins/woo-nmi-three-step/bng_gateway_functions.php on line 285

Call stack:

BNG_Custom_Payment_Gateway::receipt_page()
wp-includes/class-wp-hook.php:287
WP_Hook::apply_filters()
wp-includes/class-wp-hook.php:311
WP_Hook::do_action()
wp-includes/plugin.php:478
do_action()
wp-content/plugins/woocommerce/templates/checkout/order-receipt.php:44
include()
wp-content/plugins/woocommerce/includes/wc-core-functions.php:344
wc_get_template()
wp-content/plugins/woocommerce/includes/shortcodes/class-wc-shortcode-checkout.php:208
WC_Shortcode_Checkout::order_pay()
wp-content/plugins/woocommerce/includes/shortcodes/class-wc-shortcode-checkout.php:59
WC_Shortcode_Checkout::output()
wp-content/plugins/woocommerce/includes/class-wc-shortcodes.php:72
WC_Shortcodes::shortcode_wrapper()
wp-content/plugins/woocommerce/includes/class-wc-shortcodes.php:95
WC_Shortcodes::checkout()
wp-includes/shortcodes.php:343
do_shortcode_tag()
preg_replace_callback()
wp-includes/shortcodes.php:218
do_shortcode()
wp-includes/class-wp-hook.php:287
WP_Hook::apply_filters()
wp-includes/plugin.php:206
apply_filters()
wp-includes/post-template.php:253
the_content()
wp-content/themes/zita/template-parts/content-page.php:21
require()
wp-includes/template.php:732
load_template()
wp-includes/template.php:676
locate_template()
wp-includes/general-template.php:204
get_template_part()
wp-content/themes/zita/page.php:25
include()
wp-includes/template-loader.php:106
require_once()
wp-blog-header.php:19
require()
index.php:17

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