Sorry for the delay. I trust you've found your way around this issue, but here are our suggestions for next time:
For you to get your access token, make sure that you include all the required and correct parameters. In your case, it seems you have not correctly included your client_id and client_secret in creating a token. They should both be included in the postman 'environment' or in the parameters of your Create Token call. Happy to help guide you through this further if you get stuck.
Please double-check your credentials which you can find via My Apps & Keys on your developer dashboard https://dev.telstra.com/. You will need a TelstraDev developer account to get access to this.
For reference, kindly check this link: https://dev.telstra.com/content/messaging-api#section/Getting-Access-to-the-API
Michelle from TelstraDev