Why does the playbook fail?

Make sure that you are on ansible version >=2.1. You can check your ansible version by typing:

ansible --version

What is the username and password of the galaxy admin account ?

Username and password of the galaxy account are controlled by the variables galaxy_admin and galaxy_admin_pw and default to admin@galaxy.org and admin (Defaults are defined in group_vars/all). This should be changed in the group or host variables for the host you are working on. If you have a host in the mygroup group, you can edit group_vars/my_group and set

galaxy_admin: new_admin@email.com
galaxy_admin_pw: new_password

As with each change, run the playbook again.