You should have CURL library installed on your server. Please ask your hosting provider to install or enable it.

For developers - You can also use CURL emulation library

How to install CURL Extension Emulation Library:

Include it before any CURL functions are called. Just insert line below.

require_once('libcurlemu.inc.php');

For all osCommerce-based carts good place is top of includes/application_top.php
For X-Cart - customer\auth.php