Why cant login to couchbase-server?

I’m new with couchbase, and i install it with dpkg -i couchbase-version but when i go to localhost:8091, I can’t settings or something like that, just the login screen and about, i can not set up cluster or server or anything.

Is there a default user-password pair?

I Tried the followings:

admin-admin,root-root but i cant login.
What can be a problem, can anyone helps to me? Thanks a lot.

Ohh, and im using jessie debian 8.

When you first install Couchbase Server (i.e with no existing config) you should be presented with a setup screen, not a log-in screen unless there was already a previous installation on this machine.
Are you able to log in with the pre-existing credentials (assuming that you have access to them)?

If you wanted to install ‘from scratch’, then as well as uninstalling Couchbase Server (dpkg -r couchbase-server) you will want to delete the entire /opt/couchbase folder, before installing Couchbase Server as you did in your original post.

Please note that installing ‘from scratch’ results in removal of all previous data, so you may wish to consider backing up your data with cbbackup before doing the re-installation.