When using Firebase SDK with cloud function to send push notification, below are the errors and its description related to iOS.
"messaging/third-party-auth-error"
This error means that the authentication with the APNS failed. This is most likely due to not uploading the APNS key in the Firebase Cloud Messaging Settings.
"messaging/invalid-argument"
This error is thrown when there is an issue with the token. This can arise from an invalid or expired token. This could also arise when a token obtained from iOS simulator is used. It's better to test push notification using a iOS device. Later versions of Xcode supports push notification on simulator but needs additional configuration.