Debugging -vvv reveals that Ansible is trying to connect with no user:

ESTABLISH SSH CONNECTION FOR USER: None

Solution1:

Which causing the server to require a password. As a workaround, I passed manually root as ansible_user in an environment variable in the command:

-e "ansible_user=root"

Now it works fine:

$ ansible-playbook -e "ansible_user=root" debianservers.yml
[DEPRECATION WARNING]: DEFAULT_SUDO_USER option, In favor of Ansible Become, which is a generic framework. See become_user. , use become instead. This
feature will be removed in version 2.8. Deprecation warnings can be disabled by setting deprecation_warnings=False in ansible.cfg.

---

< PLAY [webservers] >

    \   ^__^
     \  (oo)\_______
        (__)\       )\/\
            ||----w |
            ||     ||

---

< TASK [Gathering Facts] >

    \   ^__^
     \  (oo)\_______
        (__)\       )\/\
            ||----w |
            ||     ||

ok: [51.15.238.91]

---

< TASK [Record start time of the role] >

    \   ^__^
     \  (oo)\_______
        (__)\       )\/\
            ||----w |
            ||     ||

changed: [51.15.238.91]

---

/ TASK [debian_webservers : Install (Debian/Ubuntu) Apache
\ web server] /

    \   ^__^
     \  (oo)\_______
        (__)\       )\/\
            ||----w |
            ||     ||

ok: [51.15.238.91]

---

/ TASK [debian_webservers : Wait for the a file to be
\ present] /

    \   ^__^
     \  (oo)\_______
        (__)\       )\/\
            ||----w |
            ||     ||

ok: [51.15.238.91]

---

< TASK [debian_webservers : Install vim package] >

    \   ^__^
     \  (oo)\_______
        (__)\       )\/\
            ||----w |
            ||     ||

ok: [51.15.238.91]

---

< TASK [Record end time of the role] >

    \   ^__^
     \  (oo)\_______
        (__)\       )\/\
            ||----w |
            ||     ||

changed: [51.15.238.91]

---

< PLAY RECAP >

    \   ^__^
     \  (oo)\_______
        (__)\       )\/\
            ||----w |
            ||     ||

51.15.238.91 : ok=6 changed=2 unreachable=0 failed=0

Solution2:

In ansible config file /etc/ansible/ansible.cfg

sudo_user      = ajn
#ask_sudo_pass = True
#ask_pass      = True

in the playbook: debianservers.yml

  become: yes
  become_user: ajn

from ansible_training.

In the playbook file "debianservers.yml", I've changed

become_user: root

fatal: [51.15.238.91]: FAILED! => {"changed": false, "module_stderr": "Shared connection to 51.15.238.91 closed.\r\n", "module_stdout": "sudo: a password is required\r\n", "msg": "MODULE FAILURE\nSee stdout/stderr for the exact error", "rc": 1}
to retry, use: --limit @/home/ajn/github/ansible_training/roleprj1/roles/waiting_for_events/debianservers.retry

from ansible_training.