Are you trying to evaluate a trail version of tableau server ??
If Yes -
By default it will ask you create user name and password
--Go to the command prompt as administrator to the location below
C:\Program Files (x86)\Tableau\Tableau Server\9.0\bin
Then Type Tabadmin Reset -->Enter
Go to http:localhost:80 (Refresh)
This will allow you to create a username and password to work with ..
Using a test machine and 9.0.2 verified that this will be the behavior.
In my case I had Tableau Server installed to the default location. After doing a complete uninstall/reinstall of Tableau Server was prompted to create a new admin account. Purposely created a different one from previously used one and it worked without issue.
Again though you want to be sure to do a complete uninstall of Tableau Server as outlined by the following KB article: