If you can't install the Desktop program, you can use the Online version.
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 computer or find another computer to install it on. The CD has both Windows and Mac programs on it.
Turbo Tax Refund request form
How do I request a refund for my TurboTax product?