rsyslog.service not found by Ansible during preseed late_command

The key thing to remember is that the in-target commands are executed in a chroot environment. They are not executed in a fully booted system where core processes like systemd are running and available.

Testing

I setup a playbook based on the STIG tasks in the screenshot and ran it from the preseed. I saw the same results as you.

• Playbook

---
- hosts: localhost
  gather_facts: no
  connection: local

  tasks:
    - name: check if rsyslog.service is installed
      shell: ! systemctl list-unit-files | grep "^rsyslog.service[ \t]\+"
      changed_when: False
      check_mode: no
      register: result
      failed_when: result.rc > 1

    - name: stigrule_219160_rsyslog_enable
      service:
        name: rsyslog.service
        enabled: "yes"

• Partial preseed file

d-i pkgsel/include string ansible
d-i preseed/late_command string \
    wget -P /target/ http://REDACTED/my_playbook.yml ; \
    in-target ansible-playbook -i /dev/null -b -v /my_playbook.yml

Bionic packages ansible 2.5.1 and that version of the service module appears to execute systemctl show rsyslog.service. This does not work in a chroot environment. To demonstrate, if I open a terminal in the installer environment and run in-target systemctl show rsyslog.service then the log file will show the output

in-target: Running in chroot, ignoring request: show

Potential Fix

I did find a patch in Ansible 2.3 that addresses the problem that systemctl will ignore commands when running in a chroot environment. This was only applied to the systemd module though and not the service module. I updated my playbook and this does run successfully.

---
- hosts: localhost
  gather_facts: no
  connection: local

  tasks:
    - name: check if rsyslog.service is installed
      shell: ! systemctl list-unit-files | grep "^rsyslog.service[ \t]\+"
      changed_when: False
      check_mode: no
      register: result
      failed_when: result.rc > 1

    - name: stigrule_219160_rsyslog_enable fixed
      systemd:
        name: rsyslog.service
        enabled: "yes"

Therefore, you may be able to progress further by modifying the STIG task(s) from using the service module to using the systemd module.