Open the Keychain Access app and drag that file into the System folder of that app. Download the ISRG Root X1 certificate file from. There is a workaround for the certificate, but I emphasize that I haven't tried it and don't know whether it's effective or even advisable: (I'm not sure, but possibly Sierra is the oldest system not to have an expired certificate.) If you install this you may be able to access the Catalina installer, but I have a nasty feeling that you might still be stuck with the expired certificate and be back where you started: probably worth a try. However, according again to that help page, you can download macOS Sierra 10.12 installer, which comes as a direct link to a disk image and which, oddly, is http not https. So basically you cannot install the upgrade until you have installed the upgrade. Firefox can access https sites as it uses its own certificate, but it cannot open these links (which need to open the Mac App Store). 
(secure) Safari on your Mac cannot access https sites because the security certificate has expired. Then you can install macOS Catalina 10.15
When the new owner starts up the Mac, they can use their own information to complete the setup process.According to you have to first install macOS Mojave 10.14. If you're selling, trading in or giving away your Mac, press Command-Q to exit the setup assistant without completing the setup process, then click Shut Down. When the installation is complete, your Mac might restart to a setup assistant. Your Mac might restart and show a progress bar several times, and the screen might be empty for minutes at a time. Allow installation to complete without putting your Mac to sleep or closing its lid.
If the installer offers you the choice between installing on Macintosh HD or Macintosh HD - Data, choose Macintosh HD. If the installer doesn't see your disk, or it says that it can't install on your computer or volume, you might need to erase your disk first. If the installer asks to unlock your disk, enter the password that you use to log in to your Mac. Click Continue, then follow the onscreen instructions. Reinstalling macOS won't remove your personal data. When you see a window with the option to reinstall macOS, this means your Mac has started up from Recovery.
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