If you enable parallel builds, the build system chooses the best order for building targets, based on the dependencies declared in your project. However, this disables the build order you set up in all the Target Dependencies build phases and the scheme’s Build action. This may cause your build to fail if the target dependencies aren’t declared correctly.
In the main window, choose Edit Scheme from the scheme menu.
In the scheme editor, select Build.
Select the Parallelize Build checkbox.
To disable parallel builds, uncheck the Parallelize Build checkbox.