Failure to run `openstack overcloud ffwd-upgrade run`

Issue

• While following the FFU procedure, openstack overcloud ffwd-upgrade run failed with no meaningful error message.

• /var/log/mistral/executor.log contains the following error message:

2020-05-29 14:15:40.722 3633 ERROR mistral.executors.default_executor Traceback (most recent call last):
2020-05-29 14:15:40.722 3633 ERROR mistral.executors.default_executor   File "/usr/lib/python2.7/site-packages/mistral/executors/default_executor.py", line 114, in run_action
2020-05-29 14:15:40.722 3633 ERROR mistral.executors.default_executor     result = action.run(action_ctx)
2020-05-29 14:15:40.722 3633 ERROR mistral.executors.default_executor   File "/usr/lib/python2.7/site-packages/tripleo_common/actions/ansible.py", line 415, in run
2020-05-29 14:15:40.722 3633 ERROR mistral.executors.default_executor     self.playbook]
2020-05-29 14:15:40.722 3633 ERROR mistral.executors.default_executor   File "/usr/lib/python2.7/site-packages/tripleo_common/actions/ansible.py", line 354, in playbook
2020-05-29 14:15:40.722 3633 ERROR mistral.executors.default_executor     with open(path, 'w') as playbook:
2020-05-29 14:15:40.722 3633 ERROR mistral.executors.default_executor IOError: [Errno 2] No such file or directory: u'/var/lib/mistral/9409051c-547c-48d4-ad0e-bf029da95e06/playbook.yaml'
2020-05-29 14:15:40.722 3633 ERROR mistral.executors.default_executor

• openstack overcloud fwd-upgrade prepare successfully ran.