ProjectName.build.cs

using UnrealBuildTool;

public class MyProject : ModuleRules
{
	public MyProject(ReadOnlyTargetRules Target) : base(Target)
	{
		PCHUsage = PCHUsageMode.UseExplicitOrSharedPCHs;
    
		PublicDependencyModuleNames.AddRange(new string[] { "Core", "CoreUObject", "Engine", "InputCore", "HeadMountedDisplay" });
		PublicDependencyModuleNames.AddRange(new string[] {"AchievementSystem"});
	}
}

TestUnlock.cpp

#include "AchievementSubSystem.h"

void ATestUnlock::UnlockAchievementExample()
{
	UGameInstance* GameInstance = UGameplayStatics::GetGameInstance(GetWorld());
	UAchievementSubSystem* AchievemSubsystem = GameInstance->GetSubsystem<UAchievementSubSystem>();
	AchievemSubsystem->Unlock(FName("TestAchievement"));
}