Sorry it's too late to start a 2020 online return or use the mobile app, either on Turbo Tax or the IRS.

To do a prior year return you have to buy the Desktop program here,
https://turbotax.intuit.com/personal-taxes/past-years-products/
You will need a full Windows or Mac to install it on.

If you have a simple return and want to file for free you can fill out the forms by hand. Here are some basic forms.....
Here is the 2020 1040 return

https://www.irs.gov/pub/irs-prior/f1040--2020.pdf

or if you want bigger type use 1040SR for Seniors,
https://www.irs.gov/pub/irs-prior/f1040s--2020.pdf

2020 1040 Instructions

https://www.irs.gov/pub/irs-prior/i1040gi--2020.pdf

2020 EIC and Tax Tables
https://www.irs.gov/pub/irs-prior/i1040tt--2020.pdf

Schedules 1-3
https://www.irs.gov/pub/irs-prior/f1040s1--2020.pdf
https://www.irs.gov/pub/irs-prior/f1040s2--2020.pdf
https://www.irs.gov/pub/irs-prior/f1040s3--2020.pdf

Don’t forget your state. And you will have to print and mail your returns. So be sure to attach copies of your W2s and any 1099s that have withholding on them. You have to mail federal and state in separate envelopes because they go to different places. Get a tracking number from the post office when you mail them for proof of filing.

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)

https://turbotax.intuit.com/personal-taxes/past-years-products/

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.

https://ttlc.intuit.com/community/state-taxes/help/how-do-i-contact-my-state-department-of-revenue/0... 

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.

Since you are filing a late 2020 return, the IRS will not have a 2020 AGI in the system for you.  When the 2021 software asks for your 2020 AGI so that you can e-file 2021, use “zero.”  If you cannot e-file by using zero then you will have to print, sign and mail your 2021 return.