In order to send push notifications to iOS and Android devices you need to configure a Credential that allows Twilio to send notifications to your app.
You can configure these at different places depending on which Twilio product you use:
Chat: Configure in access token
Notify: Configure default Credential in Notify Service or Binding specific one in Binding
Voice SDK: Configure in access token
Possible Causes
Credential SID is not configured correctly
Possible Solutions
If using Chat include Credential SID in access token. Check out this guide
If using Notify, either configure default Credential SID in your Service or provide the Credential SID in the Create Binding request
If using Voice SDK, include Credential SID in access token. Check out our documentation for details