An extension to file is not an extension to pay tax.
Form 8868, Application for Automatic Extension of Time to File an Exempt Organization Return, gives exempt organizations and certain governmental entities an automatic six-month extension to file their return. The extension is automatic upon timely filing - the IRS does not need to approve it in advance.
Form 8868 extends the filing deadline only. Any excise tax reported on a return covered by Form 8868, such as certain Form 990-PF or Form 5330 liabilities, is still due by the original due date. Interest continues to accrue on unpaid amounts from the original due date regardless of the extension.