Got "invalid_client" error when getting oauth access token

Followed the api documentation to create my oauth client, then I tried to use grant type client_credentials to get access token as described here

curl cmd

curl https://www.zopim.com/oauth2/token \
  -H "Content-Type: application/x-www-form-urlencoded" \
  -d 'grant_type=client_credentials&client_id={your_client_id}&client_secret={your_client_secret}' \
  -X POST

Got error all the time

{"error": "invalid_client"}

Any idea what went wrong?

BTW, currently I'm on Zendesk Suite Professional trial.