Short answer: On FIDO2 test websites (webauthn.io), the card must be registered in the same browser session. If the page was refreshed, register again. In general: use same finger as enrolled, place the finger on the sensor before the card meets the reader, hold until confirmed.
Checklist
- Use the same finger you enrolled in the app
- Place the finger on the sensor before bringing the card to the reader
- When using with demo FIDO2 websites: no refresh between registration and sign-in
See: Sign in with the card day to day, Platform support for sign-in, NFC readers for the IDEX Card for Windows and Supported services.