Here's a guide on how to implement 2FA that paypal followed along nicely:
First step: enforce the use of an authenticater app as a backup method, with no other way around it.
Step 2: allow the user to then add *only* ONE physical security key, because who'd need two of them anyway? One of the fundamental ideas behind having physical security tokens is to have at minimum two of them in the first place, but never you mind.
step 3: confirm that the key has been added to the user's account, get rid of the add security device button
step 4: tell the user they can now log in with their security key.
The result? You're using a security key that's not registered with this website.
Log in with your authenticater app or passkey, because why not use that instead? Go into settings and confirm that your security key is actually present and that paypal just decided to be a complete idiot.

🐦🔥nemo™🐦⬛ 🇺🇦🍉