You can only claim eligible medical expenses paid for a 12-month period ending in 2021. It can be any 12-month period, so you can claim unused expenses from 2020. However, you would not be able to claim anything from 2019 on your 2021 return.
If you are using TurboTax Online, you need to use Deluxe or higher to be able to claim medical expenses. Please see these TurboTax FAQs for more information:
Why did I have to upgrade my TurboTax Online edition?
Who can file for free using TurboTax?
