In Xcode 10 there seems the export option is not available for Development / Adhoc / Enterprise mode.
But for when we go through "Distribute app" and choose "iOS App Store" then it show "Export" option there.
And if I choose to proceed with any other option other than "iOS App Store" then I end up in the following screen. next
So how to export a build with Adhoc / Development mode ?
After clicking on "Distribute App" you should see the following window:
You simply need to select the required distribution method; App Store, Ad-Hoc, Enterprise or Development.
After you select "Development" and click Next you should be prompted to select additional options:
You will then select your signing options and click Next
Finally you will get an option to export your app.
Since you have selected the option to create a manifest file for over the air distribution, you need to supply the requested URLs at step 2 before you can proceed. (Alternatively, deselect that option).