Hello Lars-Erik Østerud, thanks for posting this in the community.
In Windows 10, the Security Key entry shown under Sign‑in options does not indicate that a physical USB security key has been configured on the device. It simply reflects that Windows supports FIDO and WebAuthn authentication and can work with external security devices if requested. This capability is automatically enabled the first time a WebAuthn‑capable website is used, and Facebook is one such site. In other words, Windows is not saying that a YubiKey or similar device is registered on your system, only that the system itself supports external FIDO authenticators.
Even though Windows displays USB security key support, the actual trigger for the prompt comes from Facebook. For that reason, the recommended first step from a Microsoft support point of view is to remove the passkey directly from the Facebook account rather than trying to change anything in Windows.
Sign in to Facebook using an alternative method such as your password, an SMS code, or an email verification code. Once signed in, open Facebook settings, go to Settings and Privacy, then Settings. From there, open Accounts Center, select Password and security, and then open Passkey. Remove the passkey that is associated with your Windows device. After removing it, log out of Facebook, clear the browser’s site data for facebook.com, and then restart Windows.
This process stops Facebook from requesting a passkey during sign‑in, which in turn prevents Windows 10 from prompting for a USB security key.
Kind regards,
John Oli