Endless spinning circle while "Listening" for "OK Google" voice training
What ultimately cleared this issue for me was turning OFF the Accessibility Service (in Android Settings) for my Samsung Level U Pro Bluetooth headset. I learned this after resetting my phone back to factory settings and seeing that voice retraining for Google Home and the Google app all worked fine initially after the reset, and then stopped working after I had set the Samsung Level app to give me voice notifications over my Bluetooth headset. That last act apparently set an Accessibility Service for the app to ON, which seems to conflict with Google's ability to hear my voice when I'm training it to respond to, "OK Google".