No Export option in Xcode 10 for Development/Adhoc/Enterprise

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).