Why Ansible Tower Setup Is Failing At The Task 'Pre-create system backup data file to set file mode' With 'ansible_user' is undefined' Error ?

Issue

Ansible Tower setup is failing at the task 'Pre-create system backup data file to set file mode' with 'ansible_user' is undefine' error.

TASK [backup : Pre-create system backup data file to set file mode.] ******************************************************************************************
fatal: [node1]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: 'ansible_user' is undefined\n\nThe error appears to be in '/home/test-user/ansible-tower-setup-bundle-3.6.3-1/roles/backup/tasks/download.yml': line 2, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n---\n- name: Pre-create system backup data file to set file mode.\n  ^ here\n"}
fatal: [node2]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: 'ansible_user' is undefined\n\nThe error appears to be in '/home/test-user/ansible-tower-setup-bundle-3.6.3-1/roles/backup/tasks/download.yml': line 2, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n---\n- name: Pre-create system backup data file to set file mode.\n  ^ here\n"}
fatal: [node3]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: 'ansible_user' is undefined\n\nThe error appears to be in '/home/testuser/ansible-tower-setup-bundle-3.6.3-1/roles/backup/tasks/download.yml': line 2, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n---\n- name: Pre-create system backup data file to set file mode.\n  ^ here\n"}