Here you have examples in official documentation.
https://docs.ansible.com/ansible/2.4/playbooks_reuse_includes.html
I had same error as yours after applying the aproved answer. I resolved problem by creating master playbook like this:
---
- import_playbook: master-okd.yml
- import_playbook: infra-okd.yml
- import_playbook: compute-okd.yml