Coinbase's mobile apps do not support security keys.
If a security key is activated on your account, you will not be allowed to login using Coinbase iOS or Android apps. We hope to support security keys on mobile applications in the future. However, you can use Coinbase in your mobile browsers.
If you setup a security key, you will be able to login using selected web applications. Please note, many mobile browsers do not support security keys and support depends on the browser and version.
Check with your security key manufacturer to ensure they support mobile and check with your browser developer to find out if they support WebAuthN / Fido2 standard security keys.
Third party applications may have restricted functionality.
If you setup a security key, you can log in to the 3rd-party apps using Coinbase account, but you cannot perform any actions that require 2-step authentication after login. Please contact your 3rd-party application developers to find out more about future support.