java.security.NoSuchAlgorithmException: RSA Signature not available

this is exception

Exception in thread "main" java.security.NoSuchAlgorithmException: RSA Signature not available
    at java.security.Signature.getInstance(Signature.java:229)
    at MailClient.main(MailClient.java:52)

this is my code

import java.io.*;
import java.net.*;
import java.nio.ByteBuffer;
import java.util.*;
import java.security.*;

public class MailClient {

    public String getMessage(Mail m){
        return m.message;
    }

    public static void main(String[] args) throws Exception {

        // Initialisation
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        String host = args[0];
        int port = Integer.parseInt(args[1]);
        String userid = args[2];

        while(true) {
            // connect to server
            Socket s = new Socket(host,port);
            DataInputStream dis = new DataInputStream(s.getInputStream());
            DataOutputStream dos = new DataOutputStream(s.getOutputStream());
            ObjectOutputStream oos = new ObjectOutputStream(s.getOutputStream());
            oos.flush();
            ObjectInputStream ois = new ObjectInputStream(s.getInputStream());

            // TO DO: login

            // these two lines are here just to make the supplied programs run without crashing.
            // You may want to change them, and certainly add things after them
            dos.writeUTF(userid);

            String userPrivateKeyFileName = userid + ".prv";
            // Get the key to create the signature
            ObjectInputStream keyIn = new ObjectInputStream(new FileInputStream(userPrivateKeyFileName));
            PrivateKey privateKey = (PrivateKey)keyIn.readObject();
            keyIn.close();

            // create timeStamp and random number
            long t1 = (new Date()).getTime();
            double q1 = Math.random();
            // ByteBuffer to convert to bytes later
            ByteBuffer bb = ByteBuffer.allocate(16);
            bb.putLong(t1);
            bb.putDouble(q1);

            // create signature, using timeStamp and random number as data
            Signature sig = Signature.getInstance("RSA");
            sig.initSign(privateKey);
            sig.update(bb.array());
            byte[] signature = sig.sign();

            // send data and signature
            DataOutputStream out = new DataOutputStream(s.getOutputStream());
            out.writeUTF(userid);
            out.writeLong(t1);
            out.writeDouble(q1);
            out.writeInt(signature.length);
            out.write(signature);
            out.flush();

            boolean answer = dis.readBoolean();

            //passed the verifyLogin
            if (answer)
                {
                // receive how many messages
                int numMsg = dis.readInt();
                System.out.println("You have " + numMsg + " incoming messages.");

                // TO DO: read messages
                ArrayList<Mail> msg = new ArrayList<>(numMsg);
                for(int i=0;i<numMsg;i++){
                    //@Unchecked
                    msg = (ArrayList<Mail>) ois.readObject();
                }
                while(!msg.isEmpty()){
                    //for each mail, display sender,timestamp,message
                    System.out.println(msg.get(0).sender);
                    System.out.println(msg.get(0).timestamp);
                    System.out.println(msg.get(0).message);
                    MessageDigest md = MessageDigest.getInstance("SHA-1");
                    md.update(msg.get(0).hashcash);

                    byte[] digest = md.digest();
                    boolean normalMail = msg.get(0).checkHashcash(digest);
                    if(normalMail){
                    //check each mail is original that it isn't modified
                    //receive mail
                        System.out.println(msg.get(0).message);
                        }
                    else{System.out.println("it's a spam message.");
                        System.out.println(msg.get(0).message);
                        }
                    msg.remove(0);
                }


                // send messages
                System.out.println("Do you want to send a message [Y/N]?");
                String wantToSend = br.readLine();
                if (!wantToSend.equals("Y")) {
                    dos.writeBoolean(false);
                    return ;
                }
                dos.writeBoolean(true);

                System.out.println("Enter userid of recipient:");
                String recipient = br.readLine();
                System.out.println("Type your message:");
                String message = br.readLine();

                // TO DO: send mail
                Mail m = new Mail(userid, recipient, message);
                MessageDigest md = MessageDigest.getInstance("SHA-1");
                md.update(m.hashcash);

                byte[] digest = md.digest();
                while(m.checkHashcash(digest)){
                    m.setHashcash(digest);

                }

                out.write(digest);
                out.flush();
                // send timeStamp and digest to server
                long mailTimestamp = m.timestamp.getTime();
                out.writeLong(mailTimestamp);




                oos.writeObject(m);

                }
            }


    }

}