I answered you on a previous post that you tagged onto, but I'll answer again.

You many need to REFRESH updates for 2018 for the 1040X to work correctly, I had trouble with it last month and had to do this.  Use one of these 2 methods.

Also be sure you are checking the 2018 box...they added a 2019 box as well (no clue why!) and its first in the list, so make sure you are checking the correct box.