Link for MacOS Mojave installer does not open App Store
Solution 1:
Apparently, the link is broken on PC right now (it can open on iPhone, for some reason.)
Though, since you mentioned you're running Catalina, so if you're fine with it, you can download the installer by using terminal command:
softwareupdate --fetch-full-installer --full-installer-version 10.14.6
This downloads Mojave's installer at /Applications. I suspect it won't let you downgrade Catalina to Mojave though.