Hey there, sorry this annoying glitch popped up again after you fixed it before! Let’s walk through some tried-and-true fixes that usually squash this kind of flaky Android Studio resource issue:

• Clean & Rebuild Your Project
  Corrupted build cache is a super common culprit here. Head to Build > Clean Project, wait for it to finish, then run Build > Rebuild Project. This forces the IDE to regenerate all resource IDs from scratch, which often resolves missing symbol errors.

• Invalidate Caches & Restart
  This is my go-to fix for weird IDE quirks. Go to File > Invalidate Caches..., check all the boxes in the popup, and select "Invalidate and Restart". This clears out stale cache data that might be preventing Android Studio from recognizing your layout file.

• Double-Check Layout File Details
  Make sure your page2_fragment.xml is in the correct directory (res/layout or a valid qualifier subfolder like layout-land), and that the filename is exactly lowercase—Android Studio is case-sensitive, so Page2Fragment.xml won’t match page2_fragment.

• Fix Any Broken Resources
  Even a single invalid resource (like a malformed string or missing drawable) can stop the R class from generating properly. Scan your res folder for files with red underlines and fix those errors first—this might be the hidden cause.

• Sync with Gradle Files
  Click the sync icon in the toolbar (or go to File > Sync Project with Gradle Files) to make sure your project’s Gradle config is up to date. This ensures Android Studio pulls the latest resource definitions.

• Verify Your Imports
  Accidentally importing android.R instead of your project’s own R class can cause this issue. Check the top of your Page2Fragment.java file—if you see import android.R;, delete it and make sure you’re using your app’s package-specific R import.

Your Code for Reference #

package mko.cs.stir.ac.uk.unitconv;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
public class Page2Fragment extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.page2_fragment, container, false);
    }
}

内容的提问来源于stack exchange，提问作者MrG