I am getting the following message: some sites could not be found.
You would have to check the update site settings and remove the ones that are not valid.
If this does not help you might have to check the proxy settings. Maybe you could tell us what the additional details are in the error message.
What version of KNIME do you use? If you use 3.7+ and thus 3.6- version updates are irrelevant for you, then you can simply deactivate those sites, as suggested by @mlauber71. To do that go to
File -> Preferences -> Install/Update (in the left tab) -> Available Software Sites Then consider to deactivate (tick off) the four sites that the AP can not reach in the menu on the right.
It is version 3.5.3.
You could also check if these proxy settings can help. And it seems you are mixing repositories for different versions. Maybe sort that out as well and only use the repository sufficient for your version - that is 3.5 I understand.