Authorization Failed

[ssplugin]

When I try to make payment below error occurred:
{"validationErrors":null,"response":null,"exception":{"httpStatus":"400","errorMsg":"Authorization Failed","exception":false,"errorCode":"600"}}

I don't know what is the issue because I have used the correct publishable key and privateKey with sellerId.

[Kovanija]

Can somebody help? I cannot find a solution, I am also getting 600 errorCode which means "credit card not authorized". I don't know what's wrong. I am trying to enable it within DEMO mode.