NET::ERR_CERT_AUTHORITY_INVALID

Perhaps the browser is somehow to blame? Maybe if Safari is default,

could be browser has 'missed the update capability' as macOS aged?

A newer Firefox (or other for-Mac browser) could be an answer to access

the internet and have security in doing so. ~ Browsers from third-parties

are longer lived than Safari, in part due to their independent nature.

If you haven't an alternative updated browser, to get by an error notice:

• How to download and install Firefox on Mac | Firefox Help

https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac

[There are ways to check and update Certificate Authority for browsers.]

This maybe an answer or reason for the "Err Cert Authority Invalid" here:

• El Capitan and older Mac OS X are about to have a security certificate problem | The Eclectic Light Company

https://eclecticlight.co/2021/09/21/el-capitan-and-older-mac-os-x-are-about-to-have-a-security-certificate-problem/

There was a lot going on, with browsers and security certificates, after 22/27/30 Sept.

Safari/Mac and its 'associated webkit' had some issues; other browsers had a few too.

Topic in general comes up in search engine needs fine tuned to show certain details:

https://duckduckgo.com/?t=ffab&q=how+to+update+browser+certificate+security+authority+macOS&ia=web

Temporary workarounds were proposed on some Mac-centric web sites; some worked.

..While my older Macs haven't seen a 'live Safari' update/upgrade (older macOS) mine

use 'Firefox' ESR version, and had looked into Brave browser for something different..

Good luck & happy trails!🌤❄️

Background on the certificate issue K Shaffer mentions is here:

https://scotthelme.co.uk/lets-encrypt-old-root-expiration/

You must update your Mac OS version to a newer one to get a newer version of Safari. Or, as K Shaffer says, you can try using FireFox as it handles certificates differently from Safari.