I am trying to install mercurial into Eclipse Oxygen, but I get the following error.
The Mercurial plugin site states the http address as that shown below.
https://bitbucket.org/mercurialeclipse/update-site/raw/default/
All other eclipse update sites seem to work fine so I don't think it is anything to do with having to configure a proxy.
Just wonder if anyone who know about plugins might be able to offer some help as I really need to get the plugin installed.
I am the maintainer of MercurialEclipse and I can certify that the update site works fine: installing from the update site like any user would is part of the release checklist and I did just that last November with 2.4.1. After reading your question, I have also just installed the current JDK (1.8.0_161) as well as the latest Eclipse version, Oxygen.2, on a 10-year old laptop and the installation of MercurialEclipse went smoothly.
The error message means that the Java security stack on your machine cannot link the certificate presented by bitbucket.org to a known certificate authority. The certificate chain is broken somewhere: any update site hosted on bitbucket.org will fail for you.
You need to fix the JRE or JDK that you are using.