This may actually be a SDK problem. The 4.2.4 release of Projucer and live build engine were build with the MacOSX10.11.sdk, but Xcode 8 is shipped with only MacOSX10.12.sdk. This seems fine for the Projucer to run, but not for the live build engine to compile/load/execute.
In order to fix this for the 4.2.4 release, please add either the MacOSX10.10.sdk or the MacOSX10.11.sdk to your Xcode app bunlde. You can get the SDK from old versions of Xcode (unfortunately Apple does not offer downloads for the SDKs alone). Step by step:
If you have Xcode 7 at hand, great. Otherwise, you can download it (when signed in with a Apple account) here: http://adcdownload.apple.com/Developer_Tools/Xcode_7.3/Xcode_7.3.dmg
In the Xcode app bundle, you can find the SDKs in:
Copy the missing SDK over to the same path in the Xcode 8 app bundle
Open the Projucer, do ‘Build → Clean All’ to remove cached files and check if it works now
Please let us know if this helped and which SDK you chose. Thanks again for your patience. In future releases we will detect this issue and show a warning.