10.11 is El Capitan, so you haven't provided the browser you're using.
That said, 10.11 dates from 2015 - 9 years ago - and it is no longer secure. That error is basically telling you that your present browser is not able to read the site security certificate, and may not actually be able to connect to the correct https protocol.
As said, you should firstly ensure that you're using the newest functional browser, but even then, 10.11 does not have safeguards against modern security problems, so shouldn't be in routine use without very careful and informed risk management.