If the error persists, it indicates that the Firebase C++ SDK is still not being properly linked to your Flutter Windows project. Here’s a more detailed approach to fix the issue:

### Fixing the CMake Error for Firebase Core on Windows

1. **Download the Correct Firebase C++ SDK Version:**
   - Visit [Firebase C++ SDK Releases](https://github.com/firebase/firebase-cpp-sdk/releases).
   - Download the latest stable **Windows** release (ZIP format).
   - Extract it to a known path, e.g.:
     ```
     C:/firebase_cpp_sdk
     ```

2. **Verify Directory Structure:**
   Ensure the extracted directory contains:
   ```
   C:/firebase_cpp_sdk/
     |_ include/
     |_ libs/
     |_ cmake/
     |_ bin/
   ```

3. **Modify CMakeLists.txt in Your Flutter Project:**
   Open `windows/CMakeLists.txt` in your Flutter project and add the following at the top, just before the `add_subdirectory` call:

   ```cmake
   # Point to the Firebase C++ SDK
   set(FIREBASE_CPP_SDK_DIR "C:/firebase_cpp_sdk")
   if(NOT EXISTS ${FIREBASE_CPP_SDK_DIR})
       message(FATAL_ERROR "Firebase C++ SDK not found at ${FIREBASE_CPP_SDK_DIR}")
   endif()

   # Include Firebase SDK
   add_subdirectory(${FIREBASE_CPP_SDK_DIR} ${CMAKE_BINARY_DIR}/firebase_sdk)
   ```

4. **Modify firebase_core Windows Plugin:**
   Navigate to:
   ```
   flutter/.pub-cache/hosted/pub.dev/firebase_core-<version>/windows/
   ```
   Open the `CMakeLists.txt` file and ensure that the following lines are properly set:

   ```cmake
   set(FIREBASE_CPP_SDK_DIR "C:/firebase_cpp_sdk")
   if(NOT EXISTS ${FIREBASE_CPP_SDK_DIR})
       message(FATAL_ERROR "Firebase C++ SDK not found at ${FIREBASE_CPP_SDK_DIR}")
   endif()
   add_subdirectory(${FIREBASE_CPP_SDK_DIR} firebase_sdk)
   ```

5. **Clean Flutter Cache:**
   ```bash
   flutter clean
   flutter pub cache repair
   flutter pub get
   ```

6. **Install Required Dependencies:**
   ```bash
   flutter pub add firebase_core
   flutterfire configure
   ```

7. **Rebuild Project:**
   ```bash
   flutter build windows
   ```

8. **Check for Missing CMake Files:**
   If errors persist, manually navigate to:
   ```
   C:/firebase_cpp_sdk/cmake/
   ```
   Ensure `CMakeLists.txt` exists in the correct location.

---

### If CMakeLists.txt is Missing:
1. **Re-extract Firebase SDK** – Sometimes, incomplete extractions cause missing files.
2. **Regenerate Project:**
   ```bash
   flutter create .
   flutter pub get
   ```

---

If you still encounter issues, let me know the exact Firebase C++ SDK version you're using, and I can provide a more tailored solution.