Looks like an internal build error:
FAILURE: Build failed with an exception.
* What went wrong:
Gradle build daemon disappeared unexpectedly (it may have been killed or may have crashed)
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.
at ChildProcess.whenDone (/project/cordova/node_modules/cordova-common/src/superspawn.js:169:23)
at emitTwo (events.js:106:13)
at ChildProcess.emit (events.js:191:7)
at maybeClose (internal/child_process.js:877:16)
at Process.ChildProcess._handle.onexit (internal/child_process.js:226:5)
Do you have an app on PGB that is exhibiting this error? Please share the app ID.
It fixed itself after about 20 minutes of being unable to build. I suspect this was an unannounced/silent upgrade?