Xcode on build immediately says "An unknown error occurred"

My project runs fine on android, and runs in editor on Mac ok, but when I try to build for iPhone, Unity tells me "build ok" but when it is given to Xcode, as soon as an attempt is made to build on Xcode, it tells me "An unknown error occurred." This message comes up straight away.

Google and Unity forums/answers have not been able to help me. (Or else I just missed it.)