Executing one task from a playbook using tags

I have a playbook with multiple tasks for turning on/off machines. I tried using tags for running only one task " to start VM" using the command ansible run.yaml --tags on but it throws ERROR! tags must be specified as a list. Please tell me where I have done the mistake. Thanks

---
- hosts: list
  gather_facts: no
  tasks:
  - name: start
    command: >
            virsh start {{ inventory_hostname }}
    tags: on
    delegate_to: inv
- hosts: off
  gather_facts: no
  tasks:
  - name: stop vm
    command: >
            virsh shutdown --domain {{ inventory_hostname }}
    delegate_to: inv
    tags: off

The problem is that in Ansible on is evaluated as Boolean True and off is evaluated as Boolean False. See Testing truthiness. For example

    - debug:
        msg: "{{ my_tag }} is truthy {{ my_tag is truthy }}"
      vars:
        my_tag: on

gives

  msg: True is truthy True

When you use Boolean in a tag, e.g.

    - debug:
        msg: tag on
      tags: on

Ansible complains

ERROR! tags must be specified as a list

This error is misleading. In fact, Ansible complains about the type of data. The solution is simple. Do not use Boolean as a tag, e.g. the task below works as expected

    - debug:
        msg: tag my_on
      tags: my_on

gives

shell> ansible-playbook playbook.yml --tags my_on
...
  msg: tag my_on

You can use a list in tags. But, if you put a string into the tags it will be interpreted as a single item of a list. See Tags.

Exactly as the error message says, try this instead:

---
- hosts: von
  gather_facts: no
  tasks:
  - name: start vm
    command: >
            virsh start {{ inventory_hostname }}
    tags: 
    - on
    delegate_to: sou
- hosts: voff
  gather_facts: no
  tasks:
  - name: stop vm
    command: >
            virsh shutdown --domain {{ inventory_hostname }}
    delegate_to: sou
    tags: 
    - off

Without trying your code, I suspect you may need to put the virsh command between quotation marks (") for the j2 variable substitution to work.