You’re on a fossil version of OS X, and are likely encountering a symptom of the age of that OS X version.
If this is the usual recent issue with browsing websites using KetsEncrypt certificates, this was fixed by an update that was available in macOS 10.12, and integrated in later macOS versions.
If your Mac can be upgraded—you’re checking for updates, and you have to specifically request upgrades—then you can get the fixes as part of the upgrade. Per Apple, updates are minor changes and not expected to perturb existing apps. Upgrades are larger changes, and can and variously will require app updates, upgrades, replacements, or app retirements. Apple makes updates easier to get, and upgrades are a specific request because of the app disruptions that can occur.
If your Mac is old and cannot be upgraded, then you will need to start taking over more management of your Mac and its software. In this case, that means installing the missing root certificate.
Want help here? Use 🍎 > About This Mac, and post the Mac model and Mac model year. With both the Mac model and the Mac model year, we can tell you your upgrade options. If your Mac can be upgraded, the LetsEncrypt certificate trust problem is resolved, and there are other fixes included.
If your Mac cannot be upgraded, you will have to download and install a new certificate. We can tell youmhowmto do that, if your Mac cannot be upgraded.
Have a backup before performing any upgrade.
If you want to learn more on your own: How to get old versions of macOS - Apple Support