A "couldn't resolve host" error means that the CURL software on your server was unable to lookup the IP address associated with the servername specified, when it was trying to connect to the server for communication.
Such DNS lookup failures are a result of DNS services not running reliably on your hosting server, a misconfiguration of the DNS system on your hosting server, your server is too busy to handle the requests being made of it, or the destination address is invalid.
The address is valid in this case. So, the problem is one of the other issues.
You'll have to take that issue up with your server administrator (hosting company).
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