MAAS doesn't require Juju. And if commissioning failed, you wont be able to log in with the ssh key.
To create a working server using MAAS, there is a two part process. First "Commission", then "Deploy". If Commissioning succeeds, then you can deploy, and THEN you can log in with the SSH key.
Commissioning should result in a powered-off node. So if the node is powered on and at a login prompt, you are probably seeing the previously-installed operating system. This is a common outcome when PXE fails, the old OS boots, and commissioning fails (times out) after 20mins.
Commissioning is a three phase process 1) boot the node using IPMI or BMC mechanism, 2) Use PXE to load an ephemeral operating system for inspection/testing, 3) run hardware tests and take inventory.
Since you see the node turning on and booting to the PXE prompt, I think you are okay with #1. My guess is #2 is failing -- even though it looks like it is succeeding. When PXE works, it should work quickly, you should see a message that the boot files are being downloaded, there shouldn't be any errors, and you should clearly see a message "Booting under MAAS direction".
If PXE isn't working, you may need to adjust physical networking, or the subnet/DHCP configuration in MAAS. If PXE is working, and commissioning is still failing, then you might have the wrong MAC address configured when you added the machine to MAAS. Look at the links below for a few details. If you have the wrong MAC (e.g. because you used the wrong physical port on the server), commissioning fails because MAAS doesn't know that the booting machine is the same machine as what you configured in MAAS.
How do you add a machine to MAAS with IPMI power using CLI
What are the power parameters for MAAS when using Cisco UCS blades for machines