If the iPhone Setup Tool says there is not enough free space on your driver's phone then you'll need to remove some files to continue the setup process.
Follow these steps to see how you can free space:
1. Find and open the Settings app on your driver's phone
2. Tap the "General" row
3. Tap the "iPhone Storage" row
4. On the "iPhone Storage" screen you can see what is taking up the most space on the phone. You can then uninstall apps or remove media from the phone to free enough space. You can also speak with your driver about what can be removed.
5. Once you've freed the amount of space requested by the iPhone Setup Tool, return to the Setup Tool and click "Try Again".