LNK 2001 and LNK 1120 errors in GameMode constructor

I’m following the FPS tutorial as presented here
Here are the .h and .cpp files, as requested by the tutorial.

//TestingGround.h
UCLASS()
class TESTINGGROUND_API ATestingGroundGameMode : public AGameMode
{
	GENERATED_BODY()

	virtual void StartPlay() override;
	
	ATestingGroundGameMode(const FObjectInitializer &ObjectInitializer);
	
};

**//TestingGround.cpp**

#include "TestingGround.h"
#include "TestingGroundGameMode.h"
#include "Engine.h"

ATestingGroundGameMode::ATestingGroundGameMode(const FObjectInitializer& ObjectInitializer)
: Super(ObjectInitializer)
{

}

List errors:

Error 1 error LNK2001: unresolved external symbol “private: virtual void __cdecl ATestingGroundGameMode::StartPlay(void)” (?StartPlay@ATestingGroundGameMode@@EEAAXXZ) \UE4 Projects\TestingGround\Intermediate\ProjectFiles\TestingGroundGameMode.cpp.obj TestingGround

Error 2 error LNK1120: 1 unresolved externals \UE4 Projects\TestingGround\Binaries\Win64\UE4Editor-TestingGround-248.dll 1 1 TestingGround

Error 3 error : Failed to produce item: \UE4 Projects\TestingGround\Binaries\Win64\UE4Editor-TestingGround-248.dll D:\Yarden Utilite\UE4 Projects\TestingGround\Intermediate\ProjectFiles\ERROR TestingGround

Any ideas on what is wrong?

Never mind, Forgot I didn’t implement the StartPlay() function.

ALWAYS CHECK YOU IMPLEMENT EVERY FUNCTION DELCARED.