Why am I getting a linker command error when using std::string in my plugin

Development Pipeline & Plugins Plugins
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I’m trying to create my first plugin. I’m creating something that uses the Assimp C++ library.

I have my basic plugin created and the library added and it compiles great. I started adding my first Actor class that is going to use the library. When I try to use the std::string in my plugin I get the following errors. Everything compiles until I add the following line in my cpp file. std::string filename = std::string(TCHAR_TO_UTF8(*path));

Running Mono...

Found mono via known Mono.framework path
Running installed mono, version:  Mono JIT compiler version 5.16.0.221 (2018-06/b63e5378e38 Mon Nov 19 18:08:09 EST 2018)
/Users/Shared/Epic Games/UE_4.22/Engine /Users/Shared/Epic Games/UE_4.22/Engine/Binaries/Mac
Parsing headers for SixFifteenIOPluginsEditor
  Running UnrealHeaderTool "/Users/mwallace/Dropbox/Development/Unreal Projects/SixFifteenIOPlugins/SixFifteenIOPlugins.uproject" "/Users/mwallace/Dropbox/Development/Unreal Projects/SixFifteenIOPlugins/Intermediate/Build/Mac/SixFifteenIOPluginsEditor/Development/SixFifteenIOPluginsEditor.uhtmanifest" -LogCmds="loginit warning, logexit warning, logdatabase error" -Unattended -WarningsAsErrors -installed
Reflection code generated for SixFifteenIOPluginsEditor in 10.2378211 seconds
Performing 4 actions (8 in parallel)
[2/4] Link UE4Editor-SixFifteenIOPlugins-1426.dylib
[1/4] Compile Module.RTAssetLoader.cpp
[3/4] Link UE4Editor-RTAssetLoader-0035.dylib
Undefined symbols for architecture x86_64:
  "Assimp::Importer::ReadFile(char const*, unsigned int)", referenced from:
      ARTAssetActor::openMesh(FString, int&, FString&) in Module.RTAssetLoader.cpp.o
  "Assimp::Importer::Importer()", referenced from:
      ARTAssetActor::openMesh(FString, int&, FString&) in Module.RTAssetLoader.cpp.o
  "Assimp::Importer::~Importer()", referenced from:
      ARTAssetActor::openMesh(FString, int&, FString&) in Module.RTAssetLoader.cpp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Here is my .h class
// Fill out your copyright notice in the Description page of Project Settings.

#pragma once
#include <string.h>

#include <assimp/Importer.hpp>
#include <assimp/scene.h>
#include <assimp/postprocess.h>

#include "CoreMinimal.h"
#include "GameFramework/Actor.h"
#include "RTAssetActor.generated.h"

UCLASS(ClassGroup = (Custom), meta = (BlueprintSpawnableComponent))
class RTASSETLOADER_API ARTAssetActor : public AActor
{
	GENERATED_BODY()
	
public:	
	// Sets default values for this actor's properties
	ARTAssetActor();


public:	
	// Called every frame  
  UFUNCTION(BlueprintCallable, Category = "Assimp")
  bool openMesh(FString path, int32 &SectionCount, FString &ErrorCode);
};

Here is my .cpp class

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


#include "RTAssetActor.h"
#include "assimp/DefaultLogger.hpp"
#include "assimp/Logger.hpp"
// Sets default values
ARTAssetActor::ARTAssetActor()
{
 	// Set this actor to call Tick() every frame.  You can turn this off to improve performance if you don't need it.
	PrimaryActorTick.bCanEverTick = true;

}

// Called when the game starts or when spawned
bool ARTAssetActor::openMesh(FString path, int32& SectionCount, FString& ErrorCode)
{
  Assimp::Importer importer;
  std::string filename = std::string(TCHAR_TO_UTF8(*path));
  // std::string filename(TCHAR_TO_UTF8(*path));
  const aiScene *scene = importer.ReadFile(filename, aiProcessPreset_TargetRealtime_MaxQuality);
  // const aiScene *scene = importer.ReadFile(filename, aiProcessPreset_TargetRealtime_MaxQuality);
  // if (!scene)
  // {
  //   ErrorCode = importer.GetErrorString();
  //   return false;
  // }
  // _meshCurrentlyProcessed = 0;
  // processNode(scene->mRootNode, scene);
  // SectionCount = _meshCurrentlyProcessed;
  return true;
}