If you are running Zen Cart 1.5.6 or below, note that PHP 7.3 goes out of active support at the end of November 2021.

Your best route is to upgrade to Zen Cart 1.5.7 so you can run PHP 7.4.

Upgrading instructions are provided on the documentation site, or you can place an ad in the Commercial Help Wanted forum to get someone to do it for you.

https://docs.zen-cart.com/user/upgrading/