You cannot with TurboTax and if rejected then you never filed so you do not amend but just file the original return. The 2018 software can only be install on a Mac or Windows computer.
TurboTax online and e-file for 2018 ended on Oct 15, 2019. 2018 (and earlier tax years) can now only be completed using the CD/download software installed on a Mac or Windows computer. The return must be printed, signed and mailed.
2016-2018 software can be purchased here:
See this TurboTax help article:
"How do I finish a prior-year return that I started in TurboTax Online?
If you started your 2016, 2017, or 2018 return in TurboTax Online but weren't able to file it before the shutdown, you can finish and paper-file it in the TurboTax download software for PC or Mac.
Start by purchasing and downloading the software and install it on your computer. If you're also filing a state, be sure to download and install the corresponding state program (PC instructions | Mac instructions).
1. Now sign in at TurboTax.com using the same login you used to prepare your prior-year return.
If you don't remember it, try account recovery. Still no luck? Then just start over in the TurboTax software you downloaded and skip the rest of these instructions.
2. Under Your tax returns & documents, select Download .tax file (not the PDF) for the prior year you want and save it to your computer.
3. Back in the CD/download software, select Find a Tax File, and then browse to the tax data file you downloaded in the previous step.
4. Select the file and click Open.
5 . You can now finish up in the software. Your return will need to be paper-filed, as e-filing prior years is no longer available."
@Marklucy2587 Desktop software (CD/download) can only be used on a full PC or Mac--not on a mobile device. If you are trying to prepare an earlier year return you need the desktop software to use TurboTax. The online program is only for a 2019 return--no other year.
If you do not have computer that meets the system requirements then you could use paper forms and prepare them by hand--what year do you need?
And contact customer support about the software you are unable to use. Use the word "billing" --- not refund.
To call TurboTax customer support
They are available from 5 a.m. to 5 p.m. Pacific time Monday-Friday