Did you try to switch the watch to the other arm, and see if the results are the same?
Also make sure, that it fits correctly:
...
For best results, the back of your Apple Watch needs skin contact for features like Wrist Detect, the Taptic Engine, and the electrical and optical heart sensors. Wearing your Apple Watch with the right fit—not too tight, not too loose, and with room for your skin to breathe—keeps you comfortable and let the sensors do their jobs. You may want to tighten your Apple Watch band for workouts, then loosen it when you’re done. In addition, the sensors will work only if you wear your Apple Watch on the top of your wrist.
...
copied from: Wearing your Apple Watch - Apple Support
...
Start with a good fit. Even under ideal conditions, Apple Watch may not be able to get a reliable heart rate reading every time for everybody. And for a small percentage of users, various factors may make it impossible to get any heart rate reading at all. But there are things you can do to help Apple Watch get the most consistent and best heart rate readings possible. Learn what else affects your reading.
...
copied from: Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support