"Number of open Subtasks" =0
Error: the field does not exist or you don't have permission to view it.
JIRA don't apply Field level security permissions: see https://jira.atlassian.com/browse/JRA-1330
even then this error.
this query was working on JIRA 4.1 but after transition to JIRA 5.1, its throws that errors.
project=PROJECT_KEY AND resolution = Unresolved AND
issuefunction not in parentsOf("project=PROJECT_KEY AND resolution = Unresolved") AND
issuefunction in parentsOf("project=PROJECT_KEY")
It's quite complicated but it will find all issues that have some subtasks and all of these subtasks are closed. If you want it to find open issues without subtasks as well, delete last line.