[KevinH]

That error typically means the Python version you are using has missing or outdated web certificates which are used for secure communications on the internet. These certificates expire by a certain date in case they are ever compromised.

Python 3.4 is really really old so the certificates could just be outdated. Sigil plugins can work as far back as Python 3.4 but a much more modern and still supported version of Python is best. For example Sigil on Windows and MacOS ships with Python 3.13.2.

So which platform are you using?

If Linux, which variant? Did you build this version yourself or are you using the AppImage?

The fix is possible ..
upgrading Python, or adding in new certificates, or manually downloading the epubcheck jar file and installing it in the proper place in the plugin or plugin preferences folder which is inside your Sigil Preferences folder, or ..


But knowing your platform will tell us the easiest fix.