Hi Guys, i just download it today and i am willing to explore the product.
Any recommendation or solution for the problem ?
thanks,
h3x0r !
Use the correct credentials that you set in the server.conf file?
RT(F)M and ensure you’ve set up your initial set of credentials properly in the server configuration, then use those credentials to log in with 
it depends in the Version of Graylog you use @h3x0r
Because between Version 2.x and 3.x we have changed that. You should check if the documentation you read match the version you use.