Error messae – CURL

You need to declare constant:

if (!defined('CURL_SSLVERSION_TLSv1_2')) {
define('CURL_SSLVERSION_TLSv1_2', 6);
}
curl_setopt($ch, CURLOPT_SSLVERSION, CURL_SSLVERSION_TLSv1_2);

that is common issue for example in another plugin:
https://github.com/wepay/PHP-SDK/issues/26

https://curl.se/libcurl/c/CURLOPT_SSLVERSION.html

CURL_SSLVERSION_TLSv1_2
TLS v1.2 or later (Added in 7.34.0) 

Curl version 7.34.0 was released on December 17 2013. This is almost 10 years ago.
You need to update curl in php on server.

We added define CURL_SSLVERSION_TLSv1_2 when not exist in our SDK. New version of SDK is available in plugin 2.0.31, but if your curl not supported TLS 1.2 than communication with PayU it won’t be possible.