Fatal error, when istall the viva payment plug-in

I have installed the Viva payment plug-in on my local machine and it is working. But if i have installed the plug-in live site, i couldn’t got to the wooCommerce settings page. Got the following error:

Fatal error: Uncaught Exception: Needs a 256-bit key! 27 in /home/clothing/public_html/e_retail/wp-content/plugins/woo-payment-gateway-for-vivapayments/encryption.php:27 Stack trace: #0 /home/clothing/public_html/e_retail/wp-content/plugins/woo-payment-gateway-for-vivapayments/viva-woocommerce-payment-gateway.php(207): WC_Payment_Gateway_KeyEncryption_Viva::decrypt() #1 /home/clothing/public_html/e_retail/wp-content/plugins/woo-payment-gateway-for-vivapayments/viva-woocommerce-payment-gateway.php(77): WC_Papaki_Vivapayments_Gateway->get_option() #2 /home/clothing/public_html/e_retail/wp-content/plugins/woocommerce/includes/class-wc-payment-gateways.php(94): WC_Papaki_Vivapayments_Gateway->__construct() #3 /home/clothing/public_html/e_retail/wp-content/plugins/woocommerce/includes/class-wc-payment-gateways.php(70): WC_Payment_Gateways->init() #4 /home/clothing/public_html/e_retail/wp-content/plugins/woocommerce/includes/class-wc-payment-gateways.php(43): WC_Payment_Gateways->__construct() #5 /home/clothing/public_html/e_re in /home/clothing/public_html/e_retail/wp-content/plugins/woo-payment-gateway-for-vivapayments/encryption.php on line 27

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Please help me.