android studio java package r does not exist

Question

Hello im new to android studio and to java so i tried to create button that redirects from login to register page, but i keep getting these errors even thought i did everything correctly following the tutorial

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

private Button button;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    button = findViewById(R.id.register);
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            openregister();
        }
    });

}
public void openregister (){
    Intent intent = new Intent(this,register.class);
    startActivity(intent);
}

} error i get in lines package R does not exist setContentView(R.layout.activity_main);

button = findViewById(R.id.register);

and cannot find symbol class register in

Intent intent = new Intent(this,register.class);