Why do I receive the message "The screen must be added for registration to Google Enterprise" when I want to register the G4 in Screen Control?

Since the TSx G4 runs with Google EDLA as the operating system, the registration in Screen Control is done differently. With Google EDLA, you register the screen during installation.
The following article explains how to do this:

Installation of a Prowise Touchscreen Ten G4 and registration with Prowise Screen Control

Before you begin, the Google Play Store must be activated once in Screen Control:

How do I activate the Google Play Store in Screen Control?