Android Firebase phone authentication INVALID_APP_CREDENTIAL:App validation failed

I am really new to android firebase and I have implemented the necessary libraries for Firebase Auth. I try putting a valid number, but the log says its:

W/JEJE: onVerificationFailed
com.google.firebase.FirebaseException: An internal error has occurred. [ INVALID_APP_CREDENTIAL:App validation failed ]
at com.google.android.gms.internal.nf.zzK(Unknown Source)
at com.google.android.gms.internal.og.zza(Unknown Source)
at com.google.android.gms.internal.oh.run(Unknown Source)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6176)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:888)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:778)

Here is my code:

public class MainActivity extends AppCompatActivity {

    private static String mVerificationId;
    private static PhoneAuthProvider.ForceResendingToken mResendToken;
    private static FirebaseAuth mAuth;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final EditText editText = (EditText) findViewById(R.id.phone);
        Button submit = (Button) findViewById(R.id.submit);

        final PhoneAuthProvider.OnVerificationStateChangedCallbacks mCallBacks = new PhoneAuthProvider.OnVerificationStateChangedCallbacks() {
            @Override
            public void onVerificationCompleted(PhoneAuthCredential phoneAuthCredential) {
                Log.d("JEJE", "onVerificationCompleted:" + phoneAuthCredential);

                signInWithPhoneAuthCredential(phoneAuthCredential);
            }

            @Override
            public void onVerificationFailed(FirebaseException e) {
                Log.w("JEJE", "onVerificationFailed", e);

                if (e instanceof FirebaseAuthInvalidCredentialsException) {
                    Log.d("JEJE", "INVALID REQUEST");
                } else if (e instanceof FirebaseTooManyRequestsException) {
                    Log.d("JEJE", "Too many Request");
                }
            }

            @Override
            public void onCodeSent(String s, PhoneAuthProvider.ForceResendingToken forceResendingToken) {
                super.onCodeSent(s, forceResendingToken);
                Log.d("JEJE", "onCodeSent:" + s);

                mVerificationId = s;
                mResendToken = forceResendingToken;

            }
        };

        submit.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String phoneNum = editText.getText().toString();
                Toast.makeText(MainActivity.this, phoneNum, Toast.LENGTH_SHORT).show();
                verifyPhone(phoneNum,mCallBacks);
            }

        });
    }

    private void signInWithPhoneAuthCredential(PhoneAuthCredential phoneAuthCredential) {
        mAuth.signInWithCredential(phoneAuthCredential).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if (task.isSuccessful()){
                    FirebaseUser user = task.getResult().getUser();
                }else {
                    if (task.getException() instanceof FirebaseAuthInvalidCredentialsException) {
                        // The verification code entered was invalid
                    }
                }
            }
        });
    }

    public void verifyPhone(String phoneNumber, PhoneAuthProvider.OnVerificationStateChangedCallbacks mCallbacks){
        PhoneAuthProvider.getInstance().verifyPhoneNumber(
                "+639952874699",        // Phone number to verify
                60,                 // Timeout duration
                TimeUnit.SECONDS,   // Unit of timeout
                this,               // Activity (for callback binding)
                mCallbacks);        // OnVerificationStateChangedCallback
    }

}

please tell me whats wrong thanks..