Let's say you have the following playbook, which used the firewalld module.

---
- hosts: all
  gather_facts: false
  remote_user: root
  tasks:
    - name: allow SSH in firewalld
      firewalld:
        service: ssh
        state: enabled
...

And when using the ansible-playbook command, couldn't resolve module is returned.

~]$ ansible-playbook testing.yml
ERROR! couldn't resolve module/action 'firewalld'. This often indicates a misspelling, missing collection, or incorrect module path.

The error appears to be in '/home/jeremy.canfield/testing.yml': line 21, column 5, but may
be elsewhere in the file depending on the exact syntax problem.

The offending line appears to be:


  - name: allow SSH in firewalld
    ^ here

This implies that a collection that contains the firewalld module is not installed on your control node (your Ansible server). firewalld is in the ansible.posix collection. The ansible-galaxy install collection command can be used to install the collection. In this example, the ansible.posix collection is installed.

~]$ ansible-galaxy collection install ansible.posix
Starting galaxy collection install process
Process install dependency map
Starting collection install process
Installing 'ansible.posix:1.2.0' to '/home/john.doe/.ansible/collections/ansible_collections/ansible/posix'
Downloading https://galaxy.ansible.com/download/ansible-posix-1.2.0.tar.gz to /home/john.doe/.ansible/tmp/ansible-local-15258q2alfwvq/tmpdesamf4k
ansible.posix (1.2.0) was installed successfully