I just filed my 2021 taxes and i still have to file my 2020 taxes how can i file them using this app?

No you cannot use the online program to prepare a 2020 return.

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/00/25573 

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.

To complete and file a 2020 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2020 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/

A 2020 tax return can only be printed and mailed, it cannot be e-filed.

If you want to file for free you can fill out the forms by hand. Here are some basic forms…..

Here is the IRS 2020 Form 1040
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-pdf/f1040s.pdf

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

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

Sch 1 : https://www.irs.gov/pub/irs-pdf/f1040s1.pdf
Sch 2 : https://www.irs.gov/pub/irs-pdf/f1040s2.pdf
Sch 3 : https://www.irs.gov/pub/irs-pdf/f1040s3.pdf

Then when you do file 2021 enter 0 for the 2020 AGI since you are filing 2020 late.

You have to print and mail prior year returns. 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 received the return. Don’t forget state.