Error Processing checkout, please try again

Greetings,
I received many reports from clients which had problems with completing an order.
After they fill out the informations for checkout, and choose the payment method, they click on “place an order”. After few seconds a red label pops up with the the phrase: “Error Processing checkout, please try again”.

From plesk i get the following error, (which is common for all order attempts).

AH01071: Got error 'PHP message: PHP Fatal error: Uncaught Error: Call to undefined method Nexi\\WC_Save_Order_Meta::save() in /var/www/vhosts/lostivale.eu/httpdocs/wp-content/plugins/cartasi-x-pay/src/classes/Nexi/WC_Gateway_XPay_Process_Completion.php:188\nStack trace:\n#0 /var/www/vhosts/lostivale.eu/httpdocs/wp-includes/rest-api/class-wp-rest-server.php(1171): Nexi\\WC_Gateway_XPay_Process_Completion::redirect()\n#1 /var/www/vhosts/lostivale.eu/httpdocs/wp-includes/rest-api/class-wp-rest-server.php(1018): WP_REST_Server->respond_to_request()\n#2 /var/www/vhosts/lostivale.eu/httpdocs/wp-includes/rest-api/class-wp-rest-server.php(442): WP_REST_Server->dispatch()\n#3 /var/www/vhosts/lostivale.eu/httpdocs/wp-includes/rest-api.php(410): WP_REST_Server->serve_request()\n#4 /var/www/vhosts/lostivale.eu/httpdocs/wp-includes/class-wp-hook.php(308): rest_api_loaded()\n#5 /var/www/vhosts/lostivale.eu/httpdocs/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters()\n#6 /var/www/vhosts/lostivale.eu/httpdocs/wp-includes/plugin.php(565): W...', referer: https://ecommerce.nexi.it/

I dont know which kind of problem it might be..

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