If your Mac is too old to use the CD Desktop program then it can't install the download either and you have to use the Online web version.
You can not apply the CD price to online. You need to get a refund for the CD and pay for online separately.
Contact Customer Service
5am-9pm Pacific Time Monday - Sunday
https://support.turbotax.intuit.com/contact/
What is the Turbo Tax phone number
https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632
