On the latest version of iTune it was removed the possibility to load directly an app.
So, if you have a Mac with macOS High Sierra Version 10.13.3 it is very possible your iTune will not allow you to load an app directly to an iPhone connected via USB.
Fortunately older versions of iTune can do that.
I did find on a CNET article this tip that point to this Apple article.
Basically (I'm copying from the Apple article) :
you can download this version of iTunes on your Mac, PC 32-bit or PC 64-bit* and run the installer. After installation is complete, you can continue to deploy apps with iTunes.It worked nicely.
You won’t be prompted to download new versions of iTunes after you install this version. Manually update to the latest version of iTunes when you're ready to upgrade.
So if you need/want to deploy an app (presumably under development) on a iPhone directly from your Mac, this is a nice way to do so.
To load an app
- Connect the phone via USB to the Mac
- Launch iTune on Mac
- Usually by default the selection is for Music. Force back to Apps
- Be sure to be in Apps, then File/Add to Library
Select the file to be loaded in the library
- Click on the small phone icon
- Select Apps on the menu on the left
- Select the application to install/update, then click on the Done button