Installation problem with new Ubuntu on DragonBoard410c

Hi ,
I am trying to install ubuntu core on snapdragon. It is new ubuntu . It need ssh key authentication .
While working on that particularly , I am facing problems continuously as permission denied(public key, password)

Some of the links that I have referred are
https://confluence.atlassian.com/bitbucketserver/creating-ssh-keys-776639788.html
https://help.ubuntu.com/community/SSH/OpenSSH/Keys

Ianyone could help me out, that would be great