File the oldest tax return first.
Per the IRS, you can not e-file a prior year tax return. You have to print, sign and mail it to the IRS. Additionally, the online version of TurboTax is only for the current tax filing year. So for the 2017 taxes you have to purchase and install the CD/Desktop version of TurboTax 2017. Then use that program to prepare the tax return. When done, you will have to print, sign and mail it to the IRS. Most likely you will also need to include the legal paperwork that authorizes you (or whoever) to sign the return. If a refund will be due then you will need to include form 1310 with the mailing so the IRS will make the check payable to the benificiary recipient and not to the deceased.
Get the form 1310 at https://www.irs.gov/pub/irs-pdf/f1310.pdf
For the 2018 tax year I doubt you will be able to e-file that one either, but you can try. Most likely the e-file will be rejected with a rejection message of "SSN LOCKED". If that's the case, then that indicates those who handled the final arrangements of the deceased did their job and notified the Social Security Administration of their passing. In such a case the IRS locks their SSN so you can't e-file as a method of reducing the possibility of fraud. You have no choice but to print, sign and mail the return to the IRS with the same guidance as for the 2017 tax return provided above.
Mail each year's tax return in a physically separate envelope. If you mail them in one envelope, chances are extremely high that neither one of them will be processed.
You can purchase the CD/Desktop version of TurboTax 2017 at https://turbotax.intuit.com/personal-taxes/past-years-products/
You can purchase the CD/Desktop version of TurboTax 2018 at https://turbotax.intuit.com/personal-taxes/cd-download/
