The cloud worker runs on Linux. It can read and edit an iPhone app's Swift code and its Xcode project files, but it has no Xcode, so it cannot build the app, run it, or run its tests.
The warning at the start
When the files being handed over hold an iPhone app, the start screen on the web, the Mac and the iPhone says:
The cloud can edit this iPhone app's code but can't build or run it — it's built on your Mac when the work comes back.
It shows when the files include an .xcodeproj or .xcworkspace, or a Package.swift that lists iOS among its platforms (with Swift files beside it). Projects inside node_modules, Pods, Carthage and build folders are not counted: they are someone else's. On the Mac it looks at the checkout's files; on the web and the iPhone at the discussion's project files.
It is a warning, not a stop: the run starts as usual.
What to ask for
Ask the run for work that does not need a build to check: the change itself, the reasoning, tests written for your Mac to run. A tool that only works with Xcode, such as xcodebuild or swift test against an iOS target, is not available in the cloud.