ORDS, SODA & JSON in the Database

I am trying to implement Two-legged OAuth2 following this tutorial: http://www.cloudnueva.com/blog/archives/08-2016

I am using ORDS 3.0.6. I managed to follow the instructions up to 'Obtaining An Authorization Token' where I keep getting ' 401 Unauthorized'.

Here are my steps (from schema called 'AUDIT'):

--Enable schema:
BEGIN
ORDS.ENABLE_SCHEMA(
   TRUE,
   'AUDIT');
END;
/
--Define module
begin
ORDS.define_module(
   'v1',
   'v1/');
END;
/
--define template
BEGIN
ords.define_template(
      'v1',
      'customer/');
END;
/
--define handler:
BEGIN
ORDS.define_handler(
      'v1',
      'customer/',
      'GET',
      ords.source_type_collection_feed,
      'SELECT 1 from dual');
      commit;
END;
/

I can call the service successfully using 'https://example.com/ords/audit/v1/customer'

Then I continue with the steps as follows:

--Create privilege:
begin
  ords.create_privilege(
      p_name => 'audit.customer',
      p_role_name => NULL,
      p_label => 'Customer Service Privilege',
      p_description => 'Provide access to the customer service');
  COMMIT;
END;
/
--Create validation mapping:
begin
ords.create_privilege_mapping(
      p_privilege_name => 'audit.customer',
      p_pattern => '/v1/customer');   
  commit;
end;
/
--I run validation queries on both tables 'user_ords_privileges' and 'user_ords_privilege_mappings', and they return correct result
--Create new client:
begin
oauth.create_client(
      p_name => 'ABC INC Sales System',
      p_description => 'Sales System for ABC INC.',
      p_grant_type => 'client_credentials',
      p_privilege_names => 'audit.customer',
      p_support_email => 'fred@abcinc.com');
commit;
END;
/

I ran the query below, and got the client id and secret:

select client_id,client_secret from user_ords_clients;

Now when I try to obtain a session token, (using Postman) I key in the client ID in the username field. And the client secret in the password field. I also add the pair (grant-type/client_credentials) to the body. When I try to send POST request, I get '401 Unauthorized'.

What could be wrong here? And is there any way to debug this?

Thank you