As stated in the dev guide at the Limitations section, HV SMS is not currently supported on sandbox account.
If you have a production account, and have a High Volume SMS number under that account and want to test with the a2p/batch API, you can let us know your app client id and we can move it to to the production manually. If you don't have yet a HV SMS number, pls submit a request via this form and provide the app client id.
You can also use the RingCentral Labs High Volume SMS to send messages (provided that you already have a HV SMS number)
@Phong Vu can you please move this client id q0FhTO8bRuKGN_m9AT52wQ to production so I can get bearer token for the application? I need to test this API in Production . Tia