Hi, so what do I do to fix it? it was fine until I upgraded google chrome, or was that just a coincidence !!!!
The larger issue is that you're using an operating system that is no longer supported by Apple, which might pose security risks. I ran into the same issue with my 2013 MacBook Air a few years ago that was still running Mountain Lion. The current version of Safari could no longer be installed, so I switched to Chrome which was still supporting my old operating system. But within another year or so, Google also dropped support for the old operating system.
This meant I could no longer access some websites I needed, similar to what you describe. I finally upgraded to a newer version of MacOS that would let me use current browsers. But you can continue trying to find other browsers that will work, Firefox might be one of them, I don't know.
See the other thread I linked to. It was suggested that the issue with encryption certificates affects anyone running Sierra or earlier (which includes you). I don't think there's anything you can do about that, although the owners of the affected websites should be able to fix it if they want to.
As far as "what to do", you can update to a newer version of MacOS that supports current browsers. But that could be an issue in some cases, depending on the other software you use. Old 32-bit apps will no longer run under Catalina or newer versions of MacOS. For that reason, some people have stayed with Mojave although Apple will soon stop supporting it.