Error building Player: Append is not supported

Greetings,

I receive this error when appending an already-successful build in XCode. What I don't understand is I'm not changing anything when appending, and this error shows up.

I've never used an 'old version of Xcode' as the error implicitly states.

Does anyone know how to get append working on a consistent basis?

This is a very simple project, and I'm using Playhaven and TapForTap SDK's, if that offers any insight.