Buy the desktop version for 2020 and mail the return. You can no longer get a refund for that year.
2020 is no longer supported so even if you get it installed, you may not be able to update to the full 2020 program. what's on the disk is missing updates and all 2020 forms
And...any refund for 2020 has been forfeited. Even if the tax form shows a refund, the IRS will not pay it. If you owe tax due for 2020, it is owed with late filing penalties and interest that have been adding up for several years and which will continue to add up until paid.
TurboTax no longer provides nor supports the software for 2020. There may be other software providers who have 2020, or you can use the forms to prepare by hand.
Paper forms to prepare by hand:
https://www.irs.gov/pub/irs-prior/f1040--2020.pdf
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