neowin[.]net/news/how-to-fix-the-cortana-is-not-available-error-on-windows-10-version-2004
neowin[.]net/news/how-to-install-or-uninstall-cortana-on-windows-10-may-2020-update
Cortana was never the big success that Microsoft intended, so they're repurposing it for corp IT, while slowly moving it away from Win10. In Win10 2004 Cortana is a separate app, now removed from search, rather than some built-in feature.