If you are receiving a message similar to "Not a private connection" when trying to access a website that has a valid SSL certificate, the issue is most likely related to a change made on September 30 by Let's Encrypt, a global security and encryption provider used by many thousands of websites. (They have issued over 2 billion certificates.)
This change will only affect older, outdated browsers, operating systems or hardware. The first step in seeking a solution is to run any outstanding updates for your operating system and/or browsers. In some cases, hardware may be too old to handle this change.
You can read more about this in Let's Encrypt's announcement from May 7, 2021:
In some cases, an "Advanced" option is offered by the browser that will allow you to proceed to the site inspite of the questionable security. Please use this option at your own risk.
This is the result of a change made by a global encryption provider to increase security. Individual website hosts cannot do anything to affect these circumstances because the certificates that need to be updated reside on individual local machines (rather than the web server).
For more information, take a look at TechCrunch's article: