You can’t file s 2018 return electronically right now. You also can’t use TurboTax online to file a 2018 return. You can only do that using a version of the program installed on your own computer.
if you need to file a 2018 return, your only option is to prepare at using the 2018 software, then print it out and mail it in. You can’t prepare it with TurboTax online.
then, to file a 2019 return, assuming you have prepared it with the correct information and it is not contaminated with crossover from your 2018 information, try zero as your AGI. If that doesn’t work, you will have to also print and mail in your 2019 return.