An app submitted for review has to function fully on its own — it can't depend on other unreleased apps, an external device that isn't yet available, or a companion product that doesn't exist yet to actually work.
Common with hardware-companion apps submitted before the physical product has actually shipped, leaving reviewers unable to test any real functionality.
"Your app requires companion hardware or software that is not currently available, so its core functionality could not be verified, per guideline 4.2.3."