For me the Curl command to encode the clientID and secret wasn’t working and giving the same error. So instead I just encoded them in a different command, copied that value into the Curl request and it worked.
echo -n clientId:clientSecret | base64
Printed:
encodedCharacters
curl -X POST -H "Authorization: Basic encodedCharacters" "https://zoom.us/oauth/token?grant_type=account_credentials&account_id={accountId}"