Looking into the undercloud's /var/lib/mistral/<uuid>/deploy_steps_playbook.yaml

Its failing in this section:

- hosts: Controller:overcloud
  name: Host prep steps
  become: true
  gather_facts: "{{ gather_facts | default(false) }}"
  any_errors_fatal: yes
  vars:
    bootstrap_server_id: <redacted>
    deploy_identifier: <redacted>
    enable_debug: True
    enable_puppet: True
    enable_paunch: True
    container_cli: podman
    container_log_stdout_path: /var/log/containers/stdouts
    container_healthcheck_disabled: False
    docker_puppet_debug: False
    docker_puppet_process_count: 8
    docker_puppet_mount_host_puppet: True
  tasks:
    - name: Controller Host prep block
      when:
        - tripleo_role_name == 'Controller'
      block:
        - name: Controller Host prep steps
          delegate_to: localhost
          run_once: true
          debug:
            msg: Use --start-at-task 'Controller Host prep steps' to resume from this task
        - import_tasks: Controller/host_prep_tasks.yaml

***

And this part of Controller/host_prep_tasks.yaml

- import_role:
    name: tripleo_provision_mcelog
  name: import provision_mcelog
  when: false

***

This is then running https://opendev.org/openstack/tripleo-ansible/src/branch/stable/train/tripleo_ansible/roles/tripleo_provision_mcelog/tasks/main.yml which is failing the 'Gather variables for each operating system'. There are no vars files, so this task should just be skipping.

Using the following playbook I can replicate the behaviour:

- hosts: localhost
  gather_facts: "{{ gather_facts }}"
  connection: local
  tasks:
    - name: Gather variables for each operating system
      include_vars: "{{ item }}"
      with_first_found:
        - skip: true
          files:
            - "{{ ansible_distribution | lower }}-{{ ansible_distribution_version | lower }}.yml"
            - "{{ ansible_distribution | lower }}-{{ ansible_distribution_major_version | lower }}.yml"
            - "{{ ansible_os_family | lower }}-{{ ansible_distribution_major_version | lower }}.yml"
            - "{{ ansible_distribution | lower }}.yml"
            - "{{ ansible_os_family | lower }}-{{ ansible_distribution_version.split('.')[0] }}.yml"
            - "{{ ansible_os_family | lower }}.yml"
      tags:
        - always

Example execution and output:

jpretori@jpretori-mac tripleo-heat-templates % ansible-playbook /tmp/test.yml -e gather_facts=yes
[WARNING]: No inventory was parsed, only implicit localhost is available
[WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'
[WARNING]: Found variable using reserved name: gather_facts

PLAY [localhost] *******************************************************************************************************************************************************************************************

TASK [Gathering Facts] *************************************************************************************************************************************************************************************
ok: [localhost]

TASK [Gather variables for each operating system] **********************************************************************************************************************************************************

PLAY RECAP *************************************************************************************************************************************************************************************************
localhost                  : ok=1    changed=0    unreachable=0    failed=0    skipped=1    rescued=0    ignored=0   

jpretori@jpretori-mac tripleo-heat-templates % ansible-playbook /tmp/test.yml -e gather_facts=no 
[WARNING]: No inventory was parsed, only implicit localhost is available
[WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'
[WARNING]: Found variable using reserved name: gather_facts

PLAY [localhost] *******************************************************************************************************************************************************************************************

TASK [Gather variables for each operating system] **********************************************************************************************************************************************************
fatal: [localhost]: FAILED! => 
  msg: No file was found when using first_found. Use errors='ignore' to allow this task to be skipped if no files are found

PLAY RECAP *************************************************************************************************************************************************************************************************
localhost                  : ok=0    changed=0    unreachable=0    failed=1    skipped=0    rescued=0    ignored=0

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Red Hat OpenStack Platform 16.1.4 director bug fix advisory), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2021:0817