xYz Which version of Bitwarden are you using? Only the latest beta versions of Bitwarden have passkey support on Android. I'm on Bitwarden version 2024.4.1 (versioncode 10283). You'll have to opt-in to beta releases for this to work.
If you're on the beta and it still doesn't work, could you try disabling and re-enabling the toggle for Bitwarden in Settings > Passwords & accounts > Cogwheel > Bitwarden?
I did test this just now using the latest Bitwarden Beta and registration works fine, although sign-in does not work. I'll see if I can reproduce that on stock PixelOS and then report it to Bitwarden.