I don't understand. Did you prepare and file the 2018 return Online? A current year return can still be amended online. It can prepare the Form 1040X amended return.
But if you want the data file to amend on your computer, I can tell you another way to try to download your *.tax2018 data file from your online account.
Have you been trying to download it from "Your Tax Returns & Documents"? If so, I'll tell you way to open up your return and a special way to retrieve it (hopefully, but no promise.)
