The UE5 C4668 error ('SOME_MACRO' is not defined as a preprocessor macro) occurs when your project is missing a required module or macro definition. This is often caused by forgetting to add a module dependency in your Build.cs file. To fix this, open your Build.cs file and add the missing module to PublicDependencyModuleNames. For example, if you’re working with UMG, add "UMG" to the list. This ensures the Unreal Build Tool (UBT) includes the necessary dependencies, resolving the Unreal Engine 5 module error.