I've been dealing with this issue when I run my app via debug on simulator, as soon as I upgraded to the latest Xcode 8.0 and OS version 10.11.6. Error below:
Application Settings exception, Unable to store key CognitoIdentity:IdentityId:eu-west-1:KEY, got error: -34018.
Seems it's linked to the keychain but haven't got a solution on Xamarin yet.
you should check out this bug: https://bugzilla.xamarin.com/show_bug.cgi?id=43514
It is probably the same :)