Xojo "Compilation of "xxxx" Failed

What causes this error? Thinking it might be a hung compilation previous compilation, I have closed and opened Xojo, restarted the computer, but I get this (very unhelpful) error message each time.

One of the reasons I have seen, is that an app with the same name (and/or bundle id) is already running. If you don’t see it on the dock, check in the FORCE QUIT window in case it is in the background.

Another might be you have the 32bit compile flag set in “Settings”

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That was it.
How hard is it to have an error message (or compiler check) that lets you know that your program will not run on the current platform?