With Time and Leave the system checks your balance as of the date you are requesting for. If you enter a vacation request in the future first (e.g., December), then later you enter a vacation request for a more current date (e.g, May), you may not have enough hours for your future request, and your future request may convert to LWOP.
If your future request becomes ineligible because of the request you are trying to input now, this message will appear to let you know.
You can then click the View Eligibility Details link to see which request is affected and determine the appropriate next steps.
For more information, review the How To Submit A Leave Request article.