Further to this ^^ - if you are indeed using "Managed: Use Meraki hosted accounts", this is the "Owners" list (System Manager > Owners), not the normal Meraki Administrators (Organisation > Administrators).
Therefore ensure you have created the required Owners on that page and use those credentials to authenticate on the device.
Furthermore, ensure you have sufficient SM licenses, and if you're using PDL, you have assigned the SM licenses to the correct network.