Ansible playbook with when variable not executed - ansible

Question

I have the following playbook example.yml:
- hosts: all
remote_user: administrator
become: yes
tasks:
- name: Put resolv.conf
template:
src: /home/user/resolv.conf.j2
dest: /etc/resolv.conf
backup: yes
mode: 0644
when: variable_name == "string"
The purpose is to update the resolv.conf by a customized one.
For executing it I am running:
ansible-playbook example.yml -k -u administrator --become --ask-become-pass --limit server_name -e variable_name='string'
However I get the following error:
SSH password:
SUDO password[defaults to SSH password]:
PLAY [all] *********************************************************************
TASK [setup] *******************************************************************
ok: [server_name]
TASK [Put resolv.conf] *******************************************************
fatal: [server_name]: FAILED! => {"changed": true, "failed": true, "msg": "unsupported parameter for module: when"}
to retry, use: --limit #/home/user/playbooks/example.retry
PLAY RECAP *********************************************************************
server_name : ok=1 changed=0 unreachable=0 failed=1
I have tried with different syntax by applying "()" to the variable, change between double and single commas, etc. But always the same error.
If I don't use both, variable and when condition, the task is succesfully completed.
Where is the problem?

Answer 1

Mind the padding!
- hosts: all
remote_user: administrator
become: yes
tasks:
- name: Put resolv.conf
template:
src: /home/user/resolv.conf.j2
dest: /etc/resolv.conf
backup: yes
mode: 0644
when: variable_name == "string"
when is a task property, not template's parameter.