Username and Password is incorrect..

Khurshid,

Could you login to the service web portal to verify if the credentials are correct. 

Sandbox ( use sandbox username and password )
https://service.devtest.ringcentral.com

Production ( use production username and password )
https://service.ringcentral.com

If you are still unable to log in, could you provide your app name on RingCentral so that we could reset your password on Sandbox account? 

still is same issue...
Name of my app is Funnel Drop...

Since your application is in Sandbox, below is the Environment you would need to use to make the API calls

ServerURL -> https://platform.devtest.ringcentral.com Your clientID and clientSecret could be found in the app settings. 

Looks like you did not confirm the sandbox account via the email you received. I have the confirmed the same. Could you please Login using the Sandbox credentials now and verify?