"Invalid grant_type parameter or parameter missing" for JWT Auth Sample App

Hi All,

I was tying to get access token for the box using JWT Auth Sample App but its giving an error saying that "Invalid grant_type parameter or parameter missing"
pls help me to resolve this issue.
here is the client id and secreat details - 

 clientID = "bgh5h1ha8xheb0t1wlzygzvf0rcz0sde" 

clientSecret = "REDACTED"