In order to specify an Xcode version, you’ll want to wrap the version number in quotes. The error you’re seeing occurs when the version number is not wrapped in quotes. Can you try specifying the Xcode version in your circle.yml like so?
machine:
xcode:
version: "6.3.1"
If this solves your problem, please mark this as the solution by clicking on the checkbox below.
Cheers,
Frank
EDIT: As indicated below, Xcode 6.3.1 is no longer supported. Sorry for any confusion.
One of my teammates got a notification message, though, that Xcode versions were being upgraded – 6.4 is the new default and 6.3.1 is no longer supported. Sounds like that is the issue.
I tried running a build with and without quotes on my test project and falsely attributed the cause of the error to the quotes. The real reason why one build worked and the other didn’t is because we’re still in the process of upgrading our fleet and 6.3.1 is still supported on some machines.