Can't compile fire mechanic

I cannot compile step 3.2 of this tutorial: 3 - Implementing Projectiles | Unreal Engine Documentation
I’ve tried to search it on the internet, but it seems to be a C++ error.
Build errors:

  C:\Users\My_Username\Documents\Unreal Projects\FPSProject\Binaries\Win64\UE4Editor-FPSProject-4739.dll : fatal error LNK1120: 1 unresolved externals
  FPSCharacter.cpp.obj : error LNK2019: unresolved external symbol "public: void __cdecl AFPSProjectile::FireInDirection(struct FVector const &)" (?FireInDirection@AFPSProjectile@@QEAAXAEBUFVector@@@Z) referenced in function "public: void __cdecl AFPSCharacter::Fire(void)" (?Fire@AFPSCharacter@@QEAAXXZ)

My code:

// Fill out your copyright notice in the Description page of Project Settings.

#include "FPSCharacter.h"
#include "FPSProject.h"
#include "FPSProjectile.h"
#include <Camera/Cameracomponent.h>

// Sets default values
	// Set this character to call Tick() every frame.  You can turn this off to improve performance if you don't need it
	PrimaryActorTick.bCanEverTick = true;

	// Create a first person camera component
	FPSCameraComponent = CreateDefaultSubobject<UCameraComponent>(TEXT("FirstPersonCamera"));
	// Attach the camera component to our capsule component
	// Position the camera slightly above the eyes
	FPSCameraComponent->SetRelativeLocation(FVector(0.0f, 0.0f, 50.0f + BaseEyeHeight));
	// Allow the pawn to control camera rotation
	FPSCameraComponent->bUsePawnControlRotation = true;

	// Create a first person mesh component for the owning player
	FPSMesh = CreateDefaultSubobject<USkeletalMeshComponent>(TEXT("FirstPersonMesh"));
	// Only the owning player sees this mesh
	// Attach the FPS mesh to the FPS camera
	// Disable some environmental shadowing to preserve the illusion of having a single mesh
	FPSMesh->bCastDynamicShadow = false;
	FPSMesh->CastShadow = false;

	// The owning player doesn't see the regular (third-person) body mesh

// Called when the game starts or when spawned
void AFPSCharacter::BeginPlay()

	if (GEngine)
		// Put up a debug message for five seconds. The -1 "Key" value (first argument) indicates that we will never need to update or refresh this message
		GEngine->AddOnScreenDebugMessage(-1, 5.0f, FColor::Red, TEXT("We are using FPSCharacter."));

// Called every frame
void AFPSCharacter::Tick(float DeltaTime)


// Called to bind functionality to input
void AFPSCharacter::SetupPlayerInputComponent(class UInputComponent* PlayerInputComponent)

	// Set up "movement" bindings
	PlayerInputComponent->BindAxis("MoveForward", this, &AFPSCharacter::MoveForward);
	PlayerInputComponent->BindAxis("MoveRight", this, &AFPSCharacter::MoveRight);
	// Set up "look" bindings
	PlayerInputComponent->BindAxis("Turn", this, &AFPSCharacter::AddControllerYawInput);
	PlayerInputComponent->BindAxis("LookUp", this, &AFPSCharacter::AddControllerPitchInput);
	// Set up "action" bindings
	PlayerInputComponent->BindAction("Jump", IE_Pressed, this, &AFPSCharacter::StartJump);
	PlayerInputComponent->BindAction("Jump", IE_Released, this, &AFPSCharacter::StopJump);
	PlayerInputComponent->BindAction("Fire", IE_Pressed, this, &AFPSCharacter::Fire);

void AFPSCharacter::MoveForward(float Value)
	// Find out which way is "forward" and record that the player wants to move that way
	FVector Direction = FRotationMatrix(Controller->GetControlRotation()).GetScaledAxis(EAxis::X);
	AddMovementInput(Direction, Value);

void AFPSCharacter::MoveRight(float Value)
	// Find out which way is "right" and record that the player wants to move that way
	FVector Direction = FRotationMatrix(Controller->GetControlRotation()).GetScaledAxis(EAxis::Y);
	AddMovementInput(Direction, Value);

void AFPSCharacter::StartJump()
	bPressedJump = true;

void AFPSCharacter::StopJump()
	bPressedJump = false;

void AFPSCharacter::Fire()
	// Attempt to fire a projectile
	if (ProjectileClass)
		// Get the camera transform
		FVector CameraLocation;
		FRotator CameraRotation;
		GetActorEyesViewPoint(CameraLocation, CameraRotation);

		// Transform MuzzleOffset from camera space to world space
		FVector MuzzleLocation = CameraLocation + FTransform(CameraRotation).TransformVector(MuzzleOffset);
		FRotator MuzzleRotation = CameraRotation;
		// Skew the aim to be slightly upwards
		MuzzleRotation.Pitch += 10.0f;
		UWorld* World = GetWorld();
		if (World)
			FActorSpawnParameters SpawnParams;
			SpawnParams.Owner = this;
			SpawnParams.Instigator = Instigator;
			// Spawn the projectile at the muzzle
			AFPSProjectile* Projectile = World->SpawnActor<AFPSProjectile>(ProjectileClass, MuzzleLocation, MuzzleRotation, SpawnParams);
			if (Projectile)
				// Set the projectile's initial trajectory
				FVector LaunchDirection = MuzzleRotation.Vector();


// Fill out your copyright notice in the Description page of Project Settings.

#pragma once

#include "CoreMinimal.h"
#include "GameFramework/Character.h"
#include <Camera/CameraComponent.h>
#include "FPSCharacter.generated.h"

class FPSPROJECT_API AFPSCharacter : public ACharacter

	// Sets default values for this character's properties

	// Called when the game starts or when spawned
	virtual void BeginPlay() override;

	// Called every frame
	virtual void Tick(float DeltaSeconds) override;

	// Called to bind functionality to input
	virtual void SetupPlayerInputComponent(class UInputComponent* PlayerInputComponent) override;

	// Handles input for moving forward and backward
	void MoveForward(float Value);
	// Handles input for moving right and left
	void MoveRight(float Value);
	// Sets jump flag when key is pressed
	void StartJump();
	// Clears jump flag when key is released
	void StopJump();
	// Function that handles firing projectiles
	void Fire();
	// FPS camera
	UCameraComponent* FPSCameraComponent;
	// First-person mesh (arms), visible only to the owning player
	UPROPERTY(VisibleDefaultsOnly, Category = Mesh)
	USkeletalMeshComponent* FPSMesh;
	// Gun muzzle's offset from the camera location
	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = Gameplay)
	FVector MuzzleOffset;
	// Projectile class to spawn
	UPROPERTY(EditDefaultsOnly, Category = Projectile)
	TSubclassOf<class AFPSProjectile> ProjectileClass;

I’m stupid, so sorry. I forgot to declare the FireInDirection in my projectile.cpp. I have solved it and its all fine now, just had to but some more time into it…