NET::ERR_CERT_AUTHORITY_INVALID when I open a website on my Mac

I have a 2008 iMac running El Capitan, that's the highest MacOS it can run. I followed Eau Rouge's suggestions reproduced below from https://discussions.apple.com/thread/253247723 and it resolved this issue for me on my older iMac:

Click on the link below,

https://letsencrypt.org/certificates/

Down load these two files

ISRG Root X1

Self Signed click on the der to download

And ISRG Root X2

Self signed click on der to download.

Open the Keychain Access app in your Applications> Utility folder.

In the left column highlight System.

Click on File in the menubar and click on Import Items,

select the newly downloaded files in turn and press Open.

Double-click on one of the newly imported certificates, click on the disclosure triangle

and next to When using this certificate: change Use System Defaults to Always Trust.

Do the same for the other certificate.

Restart your mac.

Make sure macOS is completely up to date. Even if you install an update, go back into Software Update and check to see if there are any more updates available. If you are using macOS 10.13 or earlier, then you will need to use a third party web browser since Safari has not updated in years. Also make sure the third party browsers are completely up to date.

You appear to be running macOS 10.11 "El Capitan." If so, there is your likely reason. Pretty much all the reports here of seeing that advisory are from posters running macOS 10.11 El Capitan or older.

A major supplier of browser security certificates has allowed some of its certs to expire. This primarily affects OS versions with outdated browsers. However, I found a few sites showing the message even with modern supported browsers (Safari 14).

Here is that cert supplier's statement on the subject:

https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/

Older devices and/or their OS versions may not support technologies that the cert's replacement uses. If you tell us your Mac's sub-model from "About this Mac" we can give your very specific recommendations regarding your possible upgrade paths. To get a newer version of Safari usually requires upgrading to a newer macOS version.

However, something to try first: If you are seeing that advisory in Safari, try FireFox. I'm been told by trusted sources that FireFox handles certs differently from other browsers and sites with the expired cert may still work.

So it is either try FireFox, which still supports El Capitan or, if your Mac supports it, upgrade to a newer macOS version.

Read and follow the instructions in Steve626 reply.

After all they are the instructions I would have given you

seeing as it was me that wrote them in the first place.