ERROR! unexpected parameter type in action: <class 'ansible.parsing.yaml.objects.AnsibleSequence'>

I was asked to create a simpefile.txt if does not exist in the directory so I created a directory first using Adhoc command then using playbook I created this:

---
- hosts: all
  become: true
  tasks:
    - name: check for file
      stat: path=/home/user/test/simplefile.txt
      register: stat_result

    - name: copy file
      file: path=/home/user/test/simplefile.txt state=touch
      when: not stat_result.stat.exists

But getting an error:

ERROR! unexpected parameter type in action: <class 'ansible.parsing.yaml.objects.AnsibleSequence'>
The error appears to be in '/projects/challenge/fresco_when/tasks/main.yml': line 2, column 3, but may be elsewhere in the file depending on the exact syntax problem.

The offending line appears to be:

---
- hosts: all
  ^ here