I searched the forum and have not seen this error.

ZenCart 1.5.7B
PHP 7.3
Elavon version: This is a payment module for the Converge payment gateway operated by Elavon. July 2017

The site owner called to tell me a customer was unable to use the module. The debug file (with some info redacted is:

[18-Feb-2021 21:48:01 UTC] Request URI: /store/{ADMIN REDACTED}/index.php?cmd=orders&page=1&oID=495&action=update_order&language=en, IP address: 76.173.4.14
#1 elavon_converge->__construct() called at [/home/www/{WEBSITE REDACTED}/store/{ADMIN REDACTED}/orders.php:269]
#2 require(/home/www/{WEBSITE REDACTED}/store/{ADMIN REDACTED}/orders.php) called at [/home/www/{WEBSITE REDACTED}/store/{ADMIN REDACTED}/index.php:11]
--> PHP Warning: Use of undefined constant MODULE_PAYMENT_ELAVON_CONVERGE_LOGIN - assumed 'MODULE_PAYMENT_ELAVON_CONVERGE_LOGIN' (this will throw an Error in a future version of PHP) in /home/www/{WEBSITE REDACTED}/store/includes/modules/payment/elavon_converge.php on line 85.

[18-Feb-2021 21:48:01 UTC] Request URI: /store/{ADMIN REDACTED}/index.php?cmd=orders&page=1&oID=495&action=update_order&language=en, IP address: 76.173.4.14
#1 elavon_converge->__construct() called at [/home/www/{WEBSITE REDACTED}/store/{ADMIN REDACTED}/orders.php:269]
#2 require(/home/www/{WEBSITE REDACTED}/store/{ADMIN REDACTED}/orders.php) called at [/home/www/{WEBSITE REDACTED}/store/{ADMIN REDACTED}/index.php:11]
--> PHP Warning: Use of undefined constant MODULE_PAYMENT_ELAVON_CONVERGE_TXNKEY - assumed 'MODULE_PAYMENT_ELAVON_CONVERGE_TXNKEY' (this will throw an Error in a future version of PHP) in /home/www/{WEBSITE REDACTED}/store/includes/modules/payment/elavon_converge.php on line 85.

[18-Feb-2021 21:48:01 UTC] Request URI: /store/{ADMIN REDACTED}/index.php?cmd=orders&page=1&oID=495&action=update_order&language=en, IP address: 76.173.4.14
#1 elavon_converge->__construct() called at [/home/www/{WEBSITE REDACTED}/store/{ADMIN REDACTED}/orders.php:269]
#2 require(/home/www/{WEBSITE REDACTED}/store/{ADMIN REDACTED}/orders.php) called at [/home/www/{WEBSITE REDACTED}/store/{ADMIN REDACTED}/index.php:11]
--> PHP Warning: Use of undefined constant MODULE_PAYMENT_ELAVON_CONVERGE_RESPONSEKEY - assumed 'MODULE_PAYMENT_ELAVON_CONVERGE_RESPONSEKEY' (this will throw an Error in a future version of PHP) in /home/www/{WEBSITE REDACTED}/store/includes/modules/payment/elavon_converge.php on line 85.

Shall I revert to an earlier PHP version or is there a replacement coming?

Thanks