Troubleshooting Siri: Icon Appears but Tasks Not Executed / Apple Watch Ultra

Did you already try the troubleshooting tips mentioned in this related article?

If Siri isn't working on your Apple Watch - Apple Support

Did you try to force restart by pressing and holding the Digital Crown and the Side button together for about 10sec, until the Apple logo comes back again?

If this does not work, unpairing the watch would be the next step:

Unpair and erase your Apple Watch - Apple Support

Just to make sure, Siri is available for the country where you bought your Apple Watch and/or iPhone?

watchOS - Feature Availability - Apple

Can you use Siri on your iPhone?

Any restrictions set up in the settings for Screen Time?

Any profile installed on your devices, or are they managed devices?

Any Focus mode active?

Try completely turning off Siri and then resetting and/or unpairing the watch then click "Skip" for "Set Up Siri" and then maybe later try and turn Siri on. Hopefully that works for you! :-)