Do you have the Hue App? If not, install that from the App Store so you can use advanced settings:

in the Hue App In Settings - Voice Assistants- Make sure you’ve configured Siri & HomeKit

The Home App might have changed due to iOS 15 now and might be requiring you to say which rooms or all rooms. At any rate, delete your scene and make a new one with a new phrase, like Goodnight or whatever - click Done. See if this helps.
- If you haven’t already updated everything , do that first on all devices; HomePods, the iOS device you used to set it up and any other devices you use the home App or Homekit in. You can also test it’s working on another device that uses Siri to be sure it’s configured correctly. (iPad, iPhone, Apple Watch, Mac, Apple TV, etc).
If it doesn’t work on HomePods, it’s an issue with that and you can first try just unplugging HomePods for a few seconds, then try a Restart or factory Reset being your last resort (stereo pairs must be ungrouped first- set up HomePod again). . Keep in mind everything should be on the same wifi network and signed in with the same Apple ID for Homekit to work successfully. If HomePod isn't responding - Apple Support