Cannot resolve symbol 'FragmentFirstBinding'

The error "Cannot resolve symbol 'FragmentFirstBinding'" typically occurs when the FragmentFirstBinding class is not generated or cannot be found. Here are a few steps you can take to resolve this issue:

1. Make sure you have the necessary data binding configurations in your project. To enable data binding, check if you have the following lines in your module-level build.gradle file:

   android {
       ...
       dataBinding {
           enabled = true
       }
   }

2. Clean and rebuild your project. Sometimes, data binding classes might not be generated properly due to build issues. Cleaning and rebuilding your project can help regenerate those classes.

3. Check if you have the correct import statements in your FragmentFirst.java file. Make sure you have the following import statement at the top of your file:

   import com.example.yourappname.databinding.FragmentFirstBinding;

4. Verify that your layout file for the first fragment (fragment_first.xml) contains the correct data binding tags. The layout file should have a root tag `<layout>` and a data variable specified within it.

   <layout>
       <data>
           <variable
               name="viewModel"
               type="com.example.yourappname.viewmodel.YourViewModel" />
       </data>
       ...
   </layout>

5. If you're using Android Studio, try invalidating caches and restarting. Go to File -> Invalidate Caches / Restart, and then rebuild your project.

By following these steps, you should be able to resolve the "Cannot resolve symbol 'FragmentFirstBinding'" error.