From the other answer I see you can generate the .ipa file fine and the issue seems to be that you can't drag and drop your file anywhere as Apple have removed the applications page from iTunes.
From doing a little of my own research I think the current best solution (although I have not tried this myself yet) is to download iTunes 12.6.3 where the applications option is still available.
Please see the following link for a guide on how to do this: https://www.macworld.com/article/3230135/software-entertainment/how-to-install-itunes-1263-and-replace-itunes-127.html
The only downside I can see is that it is no longer in support as Apple only provide support for the latest version of iTunes (12.7 at the time of writing). However I guess that's exactly what stack overflow is for!
Let me know how it goes!
EDIT:
Just to let you know I've now downloaded iTunes 12.6.3 myself and I have my apps option back! Hopefully that helps you and you can then just follow the process you used to carry out. I have also found another post similar to yours with alternative solutions too:
With Apps option removed from latest iTunes 12.7.... , how to install adhoc IPA?