If so, I imagine that this may be some proxy restriction.
In this case you would have two main solutions.
1 - Configure the network user in the KNIME proxy settings at:
File> Preferences> Network Connections.
2- Download the update site in zip format and install the missings extensions.
If your working environment has limited internet access, another option for installing KNIME extensions is available. To access this additional dialog use the File → Preferences → Install / Update → Available Software Sites command. This tool has a different interface that allows update sites to be manually specified. Clicking “Add…” will allow you to either manually specify the URL of the KNIME Update Site or use the offline (.zip) version of the Update Site to customize your KNIME Analytics Platform installation. Note: It is important not to unzip the archived KNIME Update Site file prior to specifying it as an update source for KNIME.
The tips by @natanaeldgsantos are great and you should try them, if they don’t work you might need to import the security certificate of the ssl proxy into the JRE of the KNIME AP. Take a look at this post on how that is done:
PS: I strongly advise against this whenever possible, as you will obviously not get any automated updates this way. Your issue sounds like something with your network is broken (MITM or improper certificates?!) – rather try to get this fixed!