Ansible group_vars server role

Error

TASK [application : Add lbs servers ips to hosts file] *************************
[DEPRECATION WARNING]: Skipping task due to undefined attribute, in the future
this will be a fatal error.. This feature will be removed in a future release.
Deprecation warnings can be disabled by setting deprecation_warnings=False in
ansible.cfg.

TASK [application : create a database for wordpress] ***************************
fatal: [lb01]: FAILED! => {"failed": true, "msg": "ERROR! The conditional check 'server_role == 'db'' failed. The error was: ERROR! error while evaluating conditional (server_role == 'db'): ERROR! 'server_role' is undefined\n\nThe error appears to have been in '/Users/jruels/Projects/install-wordpress-ansible/roles/application/tasks/main.yml': line 6, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n## database tasks\n- include: 02-create_databases.yml\n  ^ here\n"}

host.yml

---
- name: print all server names
  debug: msg={{ item }}
  with_items: groups['lbs'] 

inventory

[apps]
app01
app02

[dbs]
db01

[lbs]
lb01

[apps:vars]
server_role=app

[dbs:vars]
server_role=db

[lbs:vars]
server_role=lb