I have mac 10.5 not 10.11 and can't update my computer? Your disk always worked in the past with my computer.
Go to this TurboTax website to request a refund of the software purchase - https://support.turbotax.intuit.com/forms/refund-request.jsp
See this TurboTax support FAQ for Mac minimum system requirements - https://ttlc.intuit.com/questions/1899173-minimum-system-requirements-for-turbotax-mac-software