If your Apple Watch screen turns off during measurement then you need to set the Wake-Up screen time to 70 seconds.
Please follow these steps:
On WatchOS 8+
- Open the Settings app on your Apple Watch.
- Tap Display & Brightness, tap Wake Duration, then tap Wake for 70 Seconds.
On WatchOS 7, WatchOS 6, WatchOS 5.
- Go to Settings → Wake Screen on your Apple Watch;
- Find and change On Tap setting to Wake for 70 Seconds