Short answer: Registration most often fails on the gesture: the enrolled finger has to be on the sensor at the same time as the card is on the reader, and both have to stay in place until the browser confirms. Also check that a security key, not a phone, is the selected method.
Checklist
- The gesture – place your enrolled finger on the sensor at the same time as the card is on the smartcard (NFC) reader, and hold the position until the browser finishes
- Do not release early – keep finger and card in place until the confirmation appears
- Right method selected – choose security key or external device when prompted
- Card full? The card holds up to 50 passkeys. If it is full, a new registration cannot be added. See: Card models and configuration.
See: Register the card with a service, Test your card's FIDO2 function and Entra ID admin setup for IDEX cards.