Each tax year has to be filed separately using the forms for the specific tax year. They cannot be combined in any way--do not even put them in the same envelope when you mail them. Software for past years is available back to 2018. Any earlier years can only be prepared on paper forms by hand--and those refunds have been forfeited now.
The current online program can only be used for a 2021 return--no other year. And if you did not file a 2020 return you will have trouble e-filing 2021. When it asks for your 2020 AGI try using zero. If zero does not work then you have to print, sign and mail 2021.
TurboTax has software for 2018 onward. Software is no longer available from TurboTax for earlier years, nor would it be supported even if you find it on eBay, etc. Updates are not available. It is also too late to receive a tax refund for a tax year earlier than 2018---those refunds have been forfeited. BUT if you owe tax due you still have to pay with penalties and interest. There might be other software providers that have software for those years, but you will not receive refunds for the returns even if the forms show a refund.
It might be better for you to seek local paid tax help in a situation when you are several years behind in filing your tax returns. Do you even have all of the documents you will need to enter and show your income—like your W-2’s and any 1099’s?
Online preparation and e-filing for 2018 and 2019 and 2020 is permanently closed.
To file a return for a prior tax year
If you need to prepare a return for 2018, 2019 or 2020 you can purchase and download desktop software to do it, then print, sign, and mail the return(s)
You may also want to explore purchasing the software from various retailers such as Amazon, Costco, Best Buy, Walmart, Sam’s, etc.
Remember to prepare your state return as well—if you live in a state that has a state income tax.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.
Federal and state returns must be in separate envelopes and they are mailed to different addresses. Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.
Note: The desktop software you need to prepare the prior year return must be installed/downloaded to a full PC or Mac. It cannot be used on a mobile device.
If you are getting a refund, there is not a penalty for filing past the deadline. If you owe taxes, the interest/penalties will be calculated by the IRS based on how much you owe and when they receive your return and payment. The IRS will bill you for this; it will not be calculated by TurboTax.