Has the phone worked in iTunes before?
If so reboot the phone and try again, or to just get the ringtone to it for now, download this. https://dl.devmate.com/com.softorino.Waltr/WALTR.dmg
Download, open the WALTR.dmg, drag Waltr to Applications.
Next locate the ringtone on your computer, it will be in home folder/Music/iTunes/iTunes Media/Tones
Next launch Waltr, and follow the instructions in the video that will come up.