charlesbro,
It sounds like the Heart Rate app is unresponsive on your Apple Watch, which is causing the inability of providing you a reading. In this case, let's close the Heart Rate app and then restart your Apple Watch. From there, you can re-open the Heart Rate app and see if it provides you a reading. Here are the steps:
1. To close an app on your Apple Watch, press the side button, then turn the Digital Crown to locate the Heart Rate app. Swipe left on the app, then tap X.
2. Restart your Apple Watch: Restart your Apple Watch - Apple Support
Following these steps may help and we look forward to hearing back from you with an update.
Kind regards.