Unfortunately, you cannot use the 21 digit Installation Key Code (IKC) you have for the CD/Download version of TurboTax. As you purchased your copy at a retail outlet, you can request a refund for that purchase, and then you would need to pay for your Online version after you complete the return.
This link explains how to make a refund request: How do I get a refund for a TurboTax product?
