I had one of those, too about a year ago. They actually sent the form back and it was stamped with the timely received date. I actually got someone on the phone and tried to argue it was timely filed but they weren't having it. 

 But no you can't put it on 2019. You have to paper file the 1120X for 2018.

Are you sure ir=t shouldn't be carried back to 2015?