DB_USER='my_db_username'
DB_PASSWORD='my_db_password'

---
- name: Test playbook
  hosts: localhost
  connection: localhost

  tasks:
        - debug: msg="username {{ lookup('env', 'DB_USER') }} password {{ lookup('env', 'DB_PASSWORD') }}"

$ ansible-playbook playbook.yml -e @env_vars

ERROR: failed to combine variables, expected dicts but got a 'dict' and a 'str'

$ . env_vars; ansible-playbook playbook.yml

PLAY [Test playbook] **********************************************************

GATHERING FACTS ***************************************************************
ok: [localhost]

TASK: [debug msg="username  password "] ***************************************
ok: [localhost] => {
    "msg": "username  password "
}

PLAY RECAP ********************************************************************
localhost                  : ok=2    changed=0    unreachable=0    failed=0