error while returning byte array

Contrary to the other answers, I wouldn't change your code to return null if you fail to encrypt the text - I would let that failure bubble up as an exception. I wouldn't declare that your method can throw Exception either - I'd specify which exceptions it can throw. You could do this at a very fine-grained level, or in this case use GeneralSecurityException which covers all the crypto-specific exceptions you're interested in.

I'd further stop using fields unnecessarily - you don't need to change any state here.

After all this refactoring, your method would become:

public static byte[] encrypt(String unencryptedString, String k)
    throws GeneralSecurityException {
    String keySpecText = String.format("%024d", Integer.parseInt(k));
    byte[] keySpecBytes = keySpecText.getBytes(StandardCharsets.UTF_16);
    KeySpec ks = new DESedeKeySpec(keySpecBytes);
    SecretKeyFactory skf = SecretKeyFactory.getInstance(DESEDE_ENCRYPTION_SCHEME);
    Key key = skf.generateSecret(ks);
    Cipher cipher = Cipher.getInstance(DESEDE_ENCRYPTION_SCHEME);
    cipher.init(Cipher.ENCRYPT_MODE, key);
    byte[] plainText = unencryptedString.getBytes(StandardCharsets.UTF_16);
    return cipher.doFinal(plainText);
}

I'm not at all sure that this is a good way of providing a key - you're limited to 2³² keys here, which isn't great, and even if you're happy with that, why take a string for the key instead of an int? But at least the code compiles, and when it fails it will properly fail.

Declare the variable in the beginning of your scope, because in the try block it may not even exist. So you can't return smth. which may not exist.

Put it there

String encryptedString = null;
byte[] encryptedText

encrytpedText only has scope within your try block. Try this:

byte[] encrytpedText = null;
try {
    cipher.init(Cipher.ENCRYPT_MODE, key);
    byte[] plainText =
            unencryptedString/*.getBytes(UNICODE_FORMAT)*/;
    encryptedText = cipher.doFinal(plainText);
   // encryptedString = new String(Base64.encodeBase64(encryptedText));
} catch (Exception e) {
    e.printStackTrace();
}
return encryptedText;

Useful in AES all lengths 128, 192, 256 bits encryption.

Just change the KEY_LEN_IN_BITS static value.

Here we go -

private static final String ALGORITHM = "AES/CBC/PKCS5Padding";
private static int ITERATIONS = 65536;
private static SecretKey secretKey = null;
final private static short KEY_LEN_IN_BITS = (short)128;
final private static byte[] BUFFER = new byte[1024 * 64];

private static byte[] generateSalt() throws NoSuchAlgorithmException {  
    Random random = SecureRandom.getInstance("SHA1PRNG");
    byte[] salt = new byte[8];
    random.nextBytes(salt);
    return salt;
}

static ArrayList<Object> encrypt256(final FileInputStream fis, final String encryptFileTarget, final String fileTitle, final String keyVal) throws Exception{

    final ArrayList<Object> encryptInfo = new ArrayList<Object>();

    try{
        byte[] saltBytes = new byte[8];
        saltBytes = generateSalt();

        SecretKeyFactory skf = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1");
        KeySpec spec = new PBEKeySpec(keyVal.toCharArray(), saltBytes, ITERATIONS, KEY_LEN_IN_BITS);
        secretKey = skf.generateSecret(spec);
        SecretKey secret = new SecretKeySpec(secretKey.getEncoded(), "AES");//"ISO-8859-1"

        Cipher cipher = Cipher.getInstance(ALGORITHM);
        cipher.init(Cipher.ENCRYPT_MODE, secret);

        AlgorithmParameters params = cipher.getParameters ();
        byte[] initIVVector = params.getParameterSpec (IvParameterSpec.class).getIV();

        FileOutputStream fos = new FileOutputStream(encryptFileTarget, true);
        CipherOutputStream cos = new CipherOutputStream(fos, cipher);

        InputStream is = new BufferedInputStream(fis);
        int bytesRead = -1;
        int count = 0;

        while((bytesRead = is.read(BUFFER)) != -1){
            //System.out.println("bytesRead values is : " + bytesRead);
            cos.write(BUFFER, 0, bytesRead);
        }

        cos.flush();
        cos.close();
        is.close();
        fos.flush();
        fos.close();
        fis.close();

        encryptInfo.add(Hex.encodeHexString(initIVVector));
        encryptInfo.add(Hex.encodeHexString(saltBytes));

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

    return encryptInfo;
}