This article explains one of the possible causes for the below error related to native passkeys:

{"error":"invalid_grant","error_description":"Invalid passkey"}

• Passkeys

The error occurs because an incorrect value was passed for the authn_response.id parameter. The authn_response.id can be found in the POST /oauth/token call.

Encode the authn_response.id field value using base64url format (similar to the rawId value).

See the header Request Parameters in the Native Passkeys API for more details.