"ERROR: This account already exists on your device" is displayed and you cannot sign in at Google Assistant setting
You cannot sign in even when you select Sign in at Google Assistant setting during initial setup.
If you are already signed in with a Google account during initial setup, select Skip at the Google Assistant setting.
After the initial setup is finished, press the MIC/Assistant button on the remote control or select the MIC/Assistant icon on the Home screen. Google Assistant setting is displayed again, and you can finish setup without errors.
Whether Google Assistant setting is displayed depends on the language and country. For details, please refer to the following article:
Supported countries/languages and how to use the Google Assistant