When the Build Fails, in the log you attached you can locate the part under the Build Failed and it will have a file path for the UAT log that includes the error that caused the build to fail.
[2015.08.28-02.48.14:308][188]MainFrameActions: Procesamiento (Windows): Program.Main: ERROR: Exception in AutomationScripts.Automation: BUILD FAILED: Failed while running Cook for C:\Users\GUILLE\Documents\Unreal Projects\MyProject2\MyProject2.uproject; see log C:\Users\GUILLE\AppData\Roaming\Unreal Engine\AutomationTool\Logs\C+Program+Files+Epic+Games+4.8\Cook.txt