Why do I get 'Build Cancelled' message?

Hi. What is likely the reason for this message and what to do about it?

swift build
Building for debugging...
[203/203] Emitting module Leaf
Build cancelled! (45.99s)

I am trying this on Ubuntu 22.04.3 (arm) with swift (installed with swiftly):

swift --version
Swift version 5.9.2 (swift-5.9.2-RELEASE)
Target: aarch64-unknown-linux-gnu

I was attempting to add a swift-foundation package to a Vapor project on Linux only and got this unknown to me error.


OK. A full restart after a good night's sleep results now in a compilation failure. Which is better. However, I still do not understand why it did not display an error when an error was due.

(It now simply complains that Locale is ambiguous.)