Summary

On the Issue View screen after selecting Create Subtask, the user is presented an error:

"We couldn't create the subtask. The default assignee does NOT have ASSIGNABLE permission OR Unassigned issues are turned off."

Update the Project Lead and Default Assignee in project settings

As a Jira administrator:

1. Navigate to Project Settings 
2. On the Details tab, ensure that Project Lead and Default Assignee include valid entries

If the Default Assignee is not a valid assignable user, sub-tasks will not be able to be created.

In this case, update the project settings to either Unassigned or to a valid user and then Save. This should address the error and allow you to create sub-tasks as normal.