You will have to clarify what you mean by "IRS sent it back."
If you are using Turbotax Online, it can only prepare a 2021 return. If you tried to use the online web site to prepare a 2020 return, and it was rejected, that's a good thing, because if it was accepted for processing you would have worse trouble. To prepare a tax return for 2020, you have to purchase Turbotax for 2020 on a CD or as a download to install on your own Mac or PC. Look for a link for "prior year products" at the bottom of this page.
Also be aware that Turbotax can't e-file a 2020 return, it has to be mailed and the IRS is really backlogged processing paper returns. If you want to e-file a 2020 return, you will have to see a professional tax preparer, which will cost more. Professional preparers can probably e-file older returns (but get confirmation before you agree to pay), but consumer software can't, this is an IRS limitation.
