you need to edit your Build.cs and add “FliteTTSPlugin” to your public dependency: PublicDependencyModuleNames.AddRange(new string[] { "Core", "CoreUObject", "Engine", "InputCore", "FliteTTSPlugin" });
Check the proper name of the plugin Then Rebuild your project and it should be available using only #inlcude "IFliteTTSPlugin.h" and #include "FliteTTSPlugin.h"