I upgraded the iTunes 12.7 version and found that I could not install ipa as before. The new version removes the App Store and the ringtone, which makes it feel awkward. I then found that I could complete the ipa installation by dragging an ipa to any list of devices.
To get the ipa loaded drag and drop the ipa file onto the "On My Device" panel in the left hand column of iTunes and then press Sync & Done
Source: https://forums.developer.apple.com/thread/86806
Update
As the previous way seems not to work in all cases a better approach would be to use the Devices and Simulators utility from XCode
Then you can add and remove applications by clicking on the + and - sign below the applications list.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With