What are you trying to do? If you still need to file a 2018 return, you cannot use online software. Online preparation and e-filing has been permanently closed for 2018 returns since mid-October of 2019. If you did not e-file it by then, the only way to file a 2018 return now is to print, sign and mail it.
If you paid for your 2018 software last year but did not file the return, see below:
Now it is too late to use online software or to e-file. If you have any changes/corrections at all to make to your 2018 return, you now have to complete it using desktop software, which you must download to a full PC or Mac (not to a mobile device).Then you need to print, sign and mail it.
If you already paid for your online software you can ask customer support for a download of the desktop software.
They are available from 5 a.m. to 5 p.m. Pacific time Monday-Friday.
If you are getting a refund, there is no penalty for filing late. If you owe tax due, then file and pay the amount due as shown on the Form 1040, but expect a bill later from the IRS for the penalty and interest you will owe. Only the IRS will calculate this—TurboTax will not calculate it.
What are you asking about? Of course you can use the 2018 software that you purchased to do and mail file a 2018 tax return. 2018 can no longer be e-filed because the IRS does not allow it after Oct 15, 2019.
Every tax year has separate software only good for that tax year because the tax laws change. What is it that you are trying to do?