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.
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.
If you used the Desktop program last year here’s How to transfer into the Online version, expand the blue link if you used the CD/download last year
https://ttlc.intuit.com/community/entering-importing/help/how-do-i-transfer-last-year-s-return-into-...
If you used the Desktop program before you may notice several differences and not like the online version. It has less features and no Forms Mode. You can only do 1 return. Your return is only saved online so be sure to save both the .tax2020 data file and the pdf file to your computer when you are done. In case you need to amend in the future or if you need a copy to get a loan or something. You might not be able to access your online return when you need a copy.
Start an Online return here,
https://turbotax.intuit.com/personal-taxes/online/
