Cannot find local variable 'OpenCVLoader' in java Android Studio

Question

I develop mobile app that recognize math expressions. I am using opencv and tess-two library. But i take error is that Cannot find local variable 'OpenCVLoader'. Why am I getting this error. How can i fix it.Can you help me please. This is important for graduation project.

static {
    if (!OpenCVLoader.initDebug()) {
        Log.w(TAG, "Unable to load OpenCV");
    } else {
        info("OpenCV loaded");
    }

    // For OCR
    System.loadLibrary("gnustl_shared");
    System.loadLibrary("nonfree");
}

public String getOCRResult(Bitmap bitmap, AssetManager assetMana) {
    mTess = new TessBaseAPI();
    String datapath = Environment.getExternalStorageDirectory() + "/RecognizeTextOCR/";
    File dir = new File(datapath + "tessdata/");
    if (dir.exists()) {
        //dir.mkdirs();
        try {
            InputStream inStream = assetMana.open("CSDL/eng.traineddata");
            FileOutputStream outStream = new FileOutputStream(datapath + "tessdata/eng.traineddata");
            byte[] buffer = new byte[1024];
            int readCount = 0;
            while ((readCount = inStream.read(buffer)) != -1) {
                outStream.write(buffer, 0, readCount);
            }
            outStream.flush();
            outStream.close();

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    mTess.setVariable(TessBaseAPI.VAR_CHAR_WHITELIST,"aAbBcCdDeEfFgGhHiIjJkKlLmMnNoOpPqQrRsStTuUvVwWxXyYzZ1234567890',.?;/ ");
    mTess.setDebug(true);
    mTess.init(datapath, "eng"); // English
    mTess.setImage(toGrayscale(bitmap));

    String result = mTess.getUTF8Text();

    return result;
}

public Bitmap toGrayscale(Bitmap bmpOriginal)
{
    int width, height;
    height = bmpOriginal.getHeight();
    width = bmpOriginal.getWidth();

    Bitmap bmpGrayscale = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
    Canvas c = new Canvas(bmpGrayscale);
    Paint paint = new Paint();
    ColorMatrix cm = new ColorMatrix();
    cm.setSaturation(0);
    ColorMatrixColorFilter f = new ColorMatrixColorFilter(cm);
    paint.setColorFilter(f);
    c.drawBitmap(bmpOriginal, 0, 0, paint);
    return bmpGrayscale;
}