The question should be posted in AWX group or instead should be posted for your Red-hat support.
Coming to your question, you don't need to specifically define the username and password in the playbook. When you create a template, you choose the playbook, credentials (already created ones under credentials section of Ansible Tower) and other attributes for that particular template. Not sure what you are trying to achieve by trying to print the credentials though.
Thanks and Regards,
Ankit Kumar Sharma