GuzzleHttp\Exception\ConnectException: cURL error 6: Could not resolve host #1996

I'm beginner to learn it. I'm facing issue below:

Error in DB Logs

GuzzleHttp\Exception\ConnectException: cURL error 6: Could not resolve host: www-dev.drupalsite.com (see http://curl.haxx.se/libcurl/c/libcurl-errors.html ) in GuzzleHttp\Handler\CurlFactory::createRejection() (line 186 of /var/www/html/site-d8/vendor/guzzlehttp/guzzle/src/Handler/CurlFactory.php ).

Error while accessing site

The website encountered an unexpected error. Please try again later.
Note that this is local dev-site using vagrant on ubuntu (VM Virtual Box) .

I have spent 2 days (googling, asking friends etc.) on that but no luck.