There are a number of bugs with the latest versions of Xcode in the current public release of Stencyl. For the latest public release, you'll need to use, IIRC, Xcode 12 or earlier, and the version number must have two parts (for example, Xcode 12.5 is ok, Xcode 12.5.1 is not).
The subcribers-only beta builds of the next release don't have any problem with the latest versions of Xcode.
Since this isn't an issue in the latest private builds, but it seems like it hadn't been filed in the issue tracker already, I've marked this as resolved.
Issue renamed from "Can't find "instrument" or something" to "Xcode 13: cannot run program "instruments""