错误：包com.google.firebase.messaging不存在

我正在尝试构建项目，但是当我这样做时，会出现以下错误：“错误：包com.google.firebase.messaging不存在”，以及其他4种同类错误。

    package md5f64326b4609986d97810cf2ced03c9ce;


public class MyFirebaseMessagingService
extends com.google.firebase.messaging.FirebaseMessagingService
implements
    mono.android.IGCUserPeer
{
/** @hide */
public static final String __md_methods;
static {
    __md_methods = 
        "n_onMessageReceived(Lcom/google/firebase/messaging/RemoteMessage;)V:GetOnMessageReceived_Lcom_google_firebase_messaging_RemoteMessage_Handler\n" +
        "";
    mono.android.Runtime.register ("FCMClient.MyFirebaseMessagingService, LimBands.Droid, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null", MyFirebaseMessagingService.class, __md_methods);
}


public MyFirebaseMessagingService ()
{
    super ();
    if (getClass () == MyFirebaseMessagingService.class)
        mono.android.TypeManager.Activate ("FCMClient.MyFirebaseMessagingService, LimBands.Droid, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null", "", this, new java.lang.Object[] {  });
}


public void onMessageReceived (com.google.firebase.messaging.RemoteMessage p0)
{
    n_onMessageReceived (p0);
}

private native void n_onMessageReceived (com.google.firebase.messaging.RemoteMessage p0);

private java.util.ArrayList refList;
public void monodroidAddReference (java.lang.Object obj)
{
    if (refList == null)
        refList = new java.util.ArrayList ();
    refList.add (obj);
}

public void monodroidClearReferences ()
{
    if (refList != null)
        refList.clear ();
}
}

这是我的整个文件，位于obj文件夹中。我已经尝试删除bin / obj文件夹，手动删除并重新安装所有nuget软件包，更改nuget软件包版本（降级和升级），但是似乎没有任何效果。

我什至将某些文件与另一种工作方式相同的解决方案进行了比较，但没有发现任何差异。