Why do I receive "Unable to update" error when launching Fortnite mobile?

If you're getting the following error "Unable to update: Your app is currently installed and managed through another source. To update it, please use that original source, or uninstall and reinstall it via the Epic Games Store." while attempting to launch Fortnite on mobile, please try uninstalling and reinstalling Fortnite from the the official Epic Games Store app or App Store.

How do I install Fortnite on iOS or iPadOS?
How do I install the Epic Games Store on Android?