ERROR: The certificate of `url' is not trusted.
ERROR: The certificate of `url' hasn't got a known issuer.
If you are using Debian or Ubuntu, install the ca-certificates package:
$ sudo apt-get install ca-certificates
If you don't care about checking the validity of the certificate, use the --no-check-certificate option:
$ wget --no-check-certificate https://download/url
Note: The second option is not recommended because of the possibility of a man-in-the-middle attack.