菜鸟级错误。

如果你要引用插件中的头文件，那么一定需要在UE4项目名.build.cs中这里添加上插件名字：

using UnrealBuildTool;

public class 项目名 : ModuleRules
{
	public 项目名(ReadOnlyTargetRules Target) : base(Target)
	{
		PCHUsage = PCHUsageMode.UseExplicitOrSharedPCHs;
		// 这里添加上插件名字，如果找不到标题中的头文件，要在这里加上"UnLua"
		PublicDependencyModuleNames.AddRange(new string[] { "Core", "CoreUObject", "Engine", "InputCore", "WebSockets", "UnLua"});

		PrivateDependencyModuleNames.AddRange(new string[] {  });

		// Uncomment if you are using Slate UI
		// PrivateDependencyModuleNames.AddRange(new string[] { "Slate", "SlateCore" });
		
		// Uncomment if you are using online features
		// PrivateDependencyModuleNames.Add("OnlineSubsystem");

		// To include OnlineSubsystemSteam, add it to the plugins section in your uproject file with the Enabled attribute set to true
	}
}

添加之后重新生成项目或者刷新项目即可。