Xcode: Archiving a project to generate .ipa but export is not an option in Organizer
So unless I am mistaken, with Xcode 10.1 on High Sierra at least, you will not find an Export button after running Product -> Archive in the process of generating an .ipa.

When you get the pop-up Organizer, instead, you want to click on the Validate button and just follow the prompts, once successful, then you can click on the Distribute button which will direct you to a pop up prompt asking you for method of distribution.