Using this http://localhost:8080/o/oauth2/token and giving the credential details with email address and password as parameter and it works giving me the token.
reference: this link

Now i'm trying to change authentication type by email to authentication by screenname as this page says:
this link

Calling  http://localhost:8080/o/oauth2/token not working with email nor screenname and giving  me the following exceptions:

giving email as parameter: username = test@liferay.com

Application {http://application.authenticationRest.portal.ivo.ir/}AuthenticationRestApplication has thrown exception, unwinding now
org.apache.cxf.interceptor.Fault: ERR_FAILED_LOGING_IN

giving screen name as parameter: username = test

Application {http://application.authenticationRest.portal.ivo.ir/}AuthenticationRestApplication has thrown exception, unwinding now
org.apache.cxf.interceptor.Fault: No User exists with the key {companyId=20101, emailAddress=test}