You can't apply the Desktop CD/Download purchase to Online. You have to pay for Online separately. You will have to get a refund for the Desktop program. Then you can use the Online web version. Or try to upgrade your Mac or find another computer to install it on. The CD has both Windows and Mac programs on it.
Or Call in for a refund. When you fill out the from use the word BILLING ISSUES instead of the word “refund.” Contact Customer Service 5am-9pm Pacific time 7 days a week, https://support.turbotax.intuit.com/contact
They might give you a service code to use the online version.