Q:

FireBaseRecyclerAdapter problems.. Why is the setQuery telling that it cannot resolve the method with the reference and class? 

Here I pasted you both codes for my fragment + the Posts.class does anyone know why this shows me an error ?  I am new to this so I really cannot figure it out by myself some help would be great :)        
FirebaseRecyclerOptions<Posts> option = 
    new FirebaseRecyclerOptions. Builder<Posts>()
            .setQuery(postsRef, Posts.class).build();

Fragment code
package com. LiFit;

import android.content. Intent;
import android.os. Bundle;
import android.view. LayoutInflater;
import android.view. View;
import android.view. ViewGroup;
import android.widget. Button;
import android.widget. ImageButton;
import android.widget. ImageView;
import android.widget. TextView;

import androidx.annotation. NonNull;
import androidx.annotation. Nullable;
import androidx.fragment.app. Fragment;
import androidx.recyclerview.widget. LinearLayoutManager;
import androidx.recyclerview.widget. RecyclerView;

import com.firebase.ui.database. FirebaseRecyclerAdapter;
import com.firebase.ui.database. FirebaseRecyclerOptions;
import com.google.firebase.firestore. CollectionReference;
import com.google.firebase.firestore. FirebaseFirestore;
import com.google.firebase.firestore. Query;

import de.hdodenhof.circleimageview. CircleImageView;

public class HomeFragment extends Fragment {

    View myFragment;
    private ImageButton postButton;
    private RecyclerView postlist;
    CollectionReference postsRef;
    FirebaseFirestore db;

    public static ProfileFragment getInstance() {
        return new ProfileFragment();
    }

    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        myFragment = inflater.inflate(R.layout.fragment_home, container, false);
        postButton = myFragment.findViewById( R.id.postButton);
        postlist = myFragment.findViewById(R.id.postlist);
        postlist.setHasFixedSize(true);
        LinearLayoutManager lmManager = new LinearLayoutManager(getActivity());
        lmManager.setReverseLayout(true);
        lmManager.setStackFromEnd(true);
        postlist.setLayoutManager(lmManager);
        db = FirebaseFirestore.getInstance();
        postsRef = db.collection("posts");
        postButton.setOnClickListener( new View. OnClickListener() {
            @Override
            public void onClick(View v) {
                SendUserToPostActivity();
            }
        } );

        DisplayAllUsersPosts();
        return myFragment;
    }

    private void DisplayAllUsersPosts() {
        FirebaseRecyclerOptions<Posts> option = new FirebaseRecyclerOptions. Builder<Posts>().setQuery(postsRef, Posts.class).build();

        FirebaseRecyclerAdapter<Posts, PostsViewHolder> firebaseRecyclerAdapter = new FirebaseRecyclerAdapter<Posts, PostsViewHolder>(option) {
            @Override
            protected void onBindViewHolder(@NonNull PostsViewHolder holder, int position, @NonNull Posts model) {

            }

            @NonNull
            @Override
            public PostsViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
                View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.posts_layout, parent, false);
                PostsViewHolder viewHolder = new PostsViewHolder(view);
                return viewHolder;
            }
        };
    }

    public static class PostsViewHolder extends RecyclerView. ViewHolder{
        View mView;
        TextView username, postTime, postDescription;
        CircleImageView profileImage;
        ImageView postImage;
        public PostsViewHolder(@NonNull View itemView) {
            super(itemView);
            mView=itemView;
            username = mView.findViewById(R.id.post_userName);
            postTime = mView.findViewById(R.id.post_uploadTime);
            postDescription = mView.findViewById(R.id.post_description);
            profileImage = mView.findViewById(R.id.post_profileImage);
            postImage = mView.findViewById(R.id.post_image);
        }
    }

    private void SendUserToPostActivity() {
        Intent postIntent = new Intent(getActivity(), PostActivity.class);
        startActivity(postIntent);
    }
}

Posts.class
package com. LiFit;

public class Posts {
    public String description, picture, postType, profileImage, time, uid, username;

    public Posts(){

    }

    public Posts(String description, String picture, String postType, String profileImage, String time, String uid, String username) {
        this.description = description;
        this.picture = picture;
        this.postType = postType;
        this.profileImage = profileImage;
        this.time = time;
        this.uid = uid;
        this.username = username;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public String getPicture() {
        return picture;
    }

    public void setPicture(String picture) {
        this.picture = picture;
    }

    public String getPostType() {
        return postType;
    }

    public void setPostType(String postType) {
        this.postType = postType;
    }

    public String getProfileImage() {
        return profileImage;
    }

    public void setProfileImage(String profileImage) {
        this.profileImage = profileImage;
    }

    public String getTime() {
        return time;
    }

    public void setTime(String time) {
        this.time = time;
    }

    public String getUid() {
        return uid;
    }

    public void setUid(String uid) {
        this.uid = uid;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }
}

A:

I probably encountered the same problem as you. 
You're using is a Firestore, not a Realtime Database, right? 
I fixed it with the following. 
app build.gradle
// before
implementation 'com.firebaseui:firebase-ui-database: 6.2.1'

// after
implementation 'com.firebaseui:firebase-ui-firestore:6.2.1'

source:
// before
import com.firebase.ui.database. FirebaseRecyclerOptions;

// after
import com.firebase.ui.firestore. FirestoreRecyclerOptions;

