This happens when the sensor isn't sending sufficient data to the bridge. These steps should help fix it:
Sensors have to be within 75 ft of the bridge.
Make sure the light on the bridge is green. If the bridge is blinking blue or red, it is having trouble finding your Wi-Fi. Please visit this page for assistance.
Your sensor may still be asleep. When you took the sensor out of the box and gave it a knock, did you see a green light? If not, follow these steps to wake it up.
Your firewall may be blocking Notion's bridge from talking to the backend. This will cause your sensors to fail at calibration. Learn how to fix this here.
If none of this works, please contact support at 877-668-4660 or email us at firstname.lastname@example.org.