Hey there ODBasford,
Thanks so much for using Apple Support Communities.
We want to help you get Siri get working as expected, so you have easy access to your favorites mix in Apple Music.
One of the more common explanations for these types of issues is that Siri is having a hard time hearing you properly due to an issue with a microphone on your device. There are a couple ways to test this to say for sure. The first thing we'd like to have you try is temporarily disabling Bluetooth on your iPhone. This is a way to ensure that there are no Bluetooth devices actively in use on your device which may have a microphone that Siri is trying to use instead of the one built in to your device. Here is how you can do that: Use Bluetooth and Wi-Fi in Control Center - Apple Support
With Bluetooth disabled, go ahead and try the Siri command again and see if things are working as expected.
If the issue continues to occur, then we'd want to take a closer look at potential issues with your device microphones by running through the steps in this article: If the microphones on your iPhone aren't working - Apple Support. Specifically, the steps to record a Voice Memo and a video using the Camera app. When you play back that content, listen to the audio quality and see if it sounds clear and easy to hear or if it's a little muddled or difficult to make out.
Should the audio not sound clear, you may want to have your iPhone inspected in person to see if a physical repair may be necessary to get things working again. To do that, you can start with the steps in this resource: iPhone Repair & Service - Apple Support. Please note that service options will vary around the globe and hardware service is not available in every region.
We hope that this information is helpful.
Have a great day!