I believe you are referring to the freeware version, ie., DVDFab HD Decrypter. This app will no longer output an .ISO file. Owners of Platinum or Gold can no longer run the freeware installer...at least not on the same rig, so I can't verify this on my end. However, a number of users have posted this.
The free trial (Platinum or Gold) is exactly the same as the retail apps; the only difference being the free trial will stop functioning after 30 days without product activation. Platinum is fully functional.
I'm not able to connect the dots as to why you can't find the .ISO, if in fact you are selecting the .ISO icon to the far right of Target.
This is the icon that Lacky2 highlighted in yellow. It looks like a dogeared paged with a disc overlay.
Do as Lacky2 suggested.
It's a simple matter of following the file path.
In Common Settings > General, you can also untick
Delete temporary file when done, but this shouldn't make any difference when trying to create an .ISO file.