Enable caBLE on your iPhone for testing
caBLE allows a nearby device (such as your iPhone) to be used an a webauthn authenticator. Given my work on WebauthnRS I naturally wanted to test this! When I initially tried to test caBLE with webauthn via my iPhone, I recieved an error that the operation wasn't available at this time. There was no other information available.
Debugging
After some digging into Console.app, I found the log message from AuthenticationServicesAgent which stated:
"Syncing platform authenticator must be enabled to register a platform public key credential; this can be enabled in Settings > Developer."
Enabling Developer Settings
Run Xcode.app on your mac. On your iPhone close and reopen settings. Then search for "developer".
Inside of that menu enable [Syncing Platform Authenticator]{.title-ref} and [Additional Logging]{.title-ref} under [PassKit]{.title-ref}.
After that you should be able to test caBLE!