Error Redsys pago con tarjeta

Buenas foro,

Cuando selecciono pago con tarjeta en la web me arroja el siguiente error. ¿Alguien reconoce de donde puede venir el fallo?

Uncaught Error: Call to undefined function mcrypt_encrypt() in /home/tiendatr/public_html/clon/wp-content/plugins/redsys/apiRedsys/apiRedsysFinal.php on line 70
encrypt_3DES()
wp-content/plugins/redsys/apiRedsys/apiRedsysFinal.php:138
createMerchantSignature()
wp-content/plugins/redsys/class-wc-redsys.php:331
generate_redsys_form()
wp-content/plugins/redsys/class-wc-redsys.php:374
receipt_page()
wp-includes/class-wp-hook.php:288
apply_filters()
wp-includes/class-wp-hook.php:312
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:251
wc_get_template()
wp-content/plugins/woocommerce/includes/shortcodes/class-wc-shortcode-checkout.php:209
order_pay()
wp-content/plugins/woocommerce/includes/shortcodes/class-wc-shortcode-checkout.php:59
output()
wp-content/plugins/woocommerce/includes/class-wc-shortcodes.php:72
shortcode_wrapper()
wp-content/plugins/woocommerce/includes/class-wc-shortcodes.php:95
checkout()
wp-includes/shortcodes.php:325
do_shortcode_tag()
preg_replace_callback()
wp-includes/shortcodes.php:199
do_shortcode()
wp-includes/class-wp-hook.php:288
apply_filters()
wp-includes/plugin.php:206
apply_filters()
wp-includes/post-template.php:253
the_content()
wp-content/themes/venedor/page.php:24
include()
wp-includes/template-loader.php:98
require_once()
wp-blog-header.php:19
require()
index.php:17

Gracias