Cannot access API

[LKRPL]

Good evening,

we follow the guide to "make the first api call" but as result we obtained:

Exception in thread "main" com.google.api.ads.common.lib.exception.OAuthException: Credential could not be refreshed.

at com.google.api.ads.common.lib.auth.OfflineCredentials.generateCredential(OfflineCredentials.java:178)

at it.reply.fcadda.ForecastGenerator.EstimateKeywordTraffic.main(EstimateKeywordTraffic.java:56)

Caused by: com.google.api.client.auth.oauth2.TokenResponseException: 400 Bad Request

{

  "error" : "invalid_grant"

}

at com.google.api.client.auth.oauth2.TokenResponseException.from(TokenResponseException.java:105)

at com.google.api.client.auth.oauth2.TokenRequest.executeUnparsed(TokenRequest.java:287)

at com.google.api.client.auth.oauth2.TokenRequest.execute(TokenRequest.java:307)

at com.google.api.client.auth.oauth2.Credential.executeRefreshToken(Credential.java:570)

at com.google.api.client.googleapis.auth.oauth2.GoogleCredential.executeRefreshToken(GoogleCredential.java:362)

at com.google.api.client.auth.oauth2.Credential.refreshToken(Credential.java:489)

at com.google.api.ads.common.lib.auth.OAuth2Helper.callRefreshToken(OAuth2Helper.java:69)

at com.google.api.ads.common.lib.auth.OfflineCredentials.generateCredential(OfflineCredentials.java:172) 

The code used is the "Estimate keyword traffic" sample located here : "https://developers.google.com/adwords/api/docs/samples/java/optimization#estimate-keyword-traffic" 

As ads.properties we have used:

a) clientId, clientSecret (associated to this account)

b) clientCustomerId (an account created and associated to a test account)

c) developer token (associated to this account) 

d) refresh token (obtained following this url "https://github.com/googleads/googleads-java-lib/wiki/API-access-using-own-credentials-(installed-application-flow)#step-2---setting-up-the-client-library" through the test account)

Can you help us? Thanks.