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.
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.