Did you skip any steps in the setup assistant? Setting up the user is one of the steps there.
As you probably have not been able to do any configuration on your installation, it would perhaps be easiest to flash the image to the sd card again and go through the setup assistant again.
Ok, have done a reflash & reboot & now found that when I go to the Octoprint web page, the login page appeared, but after a few seconds the setup assistant appeared, so all is well.
Don't know why it didn't appear before.
Glad it worked out. The first boot on a Pi3 can take a bit longer until the web interface shows up, perhaps you did not wait long enough the first time and it did not run properly.