Cannot Activate "Translate Everything Automatically" in WPML

Overview of the issue

You may encounter that it is not possible to activate the Translate Everything Automatically feature in WPML. Instead, the screen will load indefinitely, and you’ll see this error in the console:
app.js?ver=4.6.9:425 Uncaught (in promise) TypeError: Reduce of empty array with no initial value

This issue arises specifically when all Custom Post Types (CPTs) and taxonomies, are set to Display as translated mode.

Workaround

Go to WPML > Settings page, and set the Posts or Pages post type to Translatable: only show translated items mode instead of Display as translated.