Yodel5154 I am using the Revanced Twitter (version 10.86) and I also tried using the default Google Password Manager and creating a passkey there, but I ran into the same error: “Unable to authenticate via passkey.”
I then realized that the passkey needed to be created directly on the phone and not on a browser. So I tried creating it via the Vanadium browser by signing into X. However, when I reached the passkey creation step, it kept asking for a PIN with the prompt: “X needs to verify it’s you” “Use your screen lock.” I entered my screen lock, but it didn’t work. I changed my screen lock and restarted the phone and still didn’t work.
As a result, I decided I had to create the passkey from the official app on the Play Store. So when I installed the X app (using a Private Space) and tried to log in, I received the AttestationDenied error. Lol.
TL;DR It seems you need to have an existing passkey already associated with your Android device. If you don’t have an existing passkey, you’re stuck because logging in via the official app to create one fails with the AttestationDenied error too.