@simon-joshua-chang asked: "I bought TurboTax deluxe for download. But my operating system is too old, can I use the license code for TurboTax online?"
No, you cannot use a desktop software license code in the Online product. You should be able to get a refund for the desktop software purchase. Did you buy it from a retailer or direct from TurboTax?
If you bought it from an authorized retailer, check with the retailer to see if they will refund it. If they won't do it, TurboTax will still honor it under their 60-day unconditional moneyback guarantee.
FAQ: How do I request a refund for my TurboTax product?
https://ttlc.intuit.com/turbotax-support/en-us/help-article/intuit-refunds/request-refund-turbotax-product/L834M6uyW_US_en_US