Subtasks do not have the same lifecycle as user stories. So it stands to reason that the statuses for a subtasks will not always be the same as for user stories.

However JIRA won't let me close a sprint unless the subtasks of user stories have a status that's the same as one of the statuses of the user story itself.

For example, say I have a user story workflow of Ready | In Progress | Ready for QA | In QA | In Review | Done. That's fine.

Then say I create a subtask workflow of To Do | In Progress | Approved | Closed.

If I have any stories which have subtasks in a state of "Approved" or "Closed," JIRA will not allow me to close the sprint. Instead I get the message,  "Sprint cannot be completed as there are incomplete subtasks on the following issues:"

This is bizarre. Why allow people to modify workflows for different issue types and subtypes if this then prevents the valid closing of a sprint?