Credential SIDs are used to determine which Credential resource to use to send a given push notification (APNS, FCM or GCM). For each delivery attempt the system tries to select a Credential SID depending on the API request and configuration. This error indicates that the system did not find any available Credential SID to use and hence failed to send the delivery attempt.
Configuration is slightly different depending on which Twilio API and product you are using.
Notify: Credential SID can be specified in the Notify Service or in the Binding. This error indicates neither of those were specified.
Verify Push: Credential SID can be specified in the Verify Service in the push configuration. This error indicates the service doesn't have this configuration.
Chat, Voice, Verify, Lookup: Credential SID can be specified in the Twilio Access Token used to register for push notifications. This error indicates that the token missed that information.