No, the play function is the least of my problems now 
I need to find out why the 2019 model is acting lke this.
Bumping this, I’m seeing the exact same error. The YAML is fine, but I can’t even get Plex to play when I manually browse to the Plex device, browse media, and click play on anything. Throws the error:
homeassistant.exceptions.HomeAssistantError: Client is not currently accepting playback controls: Plex (Plex for Apple TV - Apple TV)
Seems like you’re hitting a frustrating roadblock there. Have you tried troubleshooting with Plex support? They might have some insights. Meanwhile, any luck finding a workaround? And hey, if you need a distraction, any good movies out rn that you’re itching to watch? Always a good backup plan for tech hiccups!