I've had the same issue when trying to load on rpi3.
I always get a password prompt even though SSO is setup with SSH public key (RSA) on Ubuntu One and private key on SSH client systems (classic Ubuntu 16.04).

Here's the debug (-v) output from the ssh client session. It falls through to password method :-(.

xxxxxx@u1604:~$ ssh -v yyyyyy@10.0.80.145
OpenSSH_7.2p2 Ubuntu-4ubuntu2.1, OpenSSL 1.0.2g  1 Mar 2016
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug1: Connecting to 10.0.80.145 [10.0.80.145] port 22.
debug1: Connection established.
debug1: identity file /home/xxxxxx/.ssh/id_rsa type 1
debug1: key_load_public: No such file or directory
debug1: identity file /home/xxxxxx/.ssh/id_rsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/xxxxxx/.ssh/id_dsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/xxxxxx/.ssh/id_dsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/xxxxxx/.ssh/id_ecdsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/xxxxxx/.ssh/id_ecdsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/xxxxxx/.ssh/id_ed25519 type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/xxxxxx/.ssh/id_ed25519-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_7.2p2 Ubuntu-4ubuntu2.1
debug1: Remote protocol version 2.0, remote software version OpenSSH_7.2p2 Ubuntu-4ubuntu2.1
debug1: match: OpenSSH_7.2p2 Ubuntu-4ubuntu2.1 pat OpenSSH* compat 0x04000000
debug1: Authenticating to 10.0.80.145:22 as 'yyyyyy'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: <email address hidden>
debug1: kex: host key algorithm: ecdsa-sha2-nistp256
debug1: kex: server->client cipher: <email address hidden> MAC: <implicit> compression: none
debug1: kex: client->server cipher: <email address hidden> MAC: <implicit> compression: none
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ecdsa-sha2-nistp256 SHA256:WGx4rmOgFW1p7Fr5UGQmOi8V/8azjAgBZLS11q8FBWk
debug1: Host '10.0.80.145' is known and matches the ECDSA host key.
debug1: Found key in /home/xxxxxx/.ssh/known_hosts:1
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_EXT_INFO received
debug1: kex_input_ext_info: server-sig-algs=<rsa-sha2-256,rsa-sha2-512>
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,password
debug1: Next authentication method: publickey
debug1: Offering RSA public key: /home/xxxxxx/.ssh/id_rsa
debug1: Authentications that can continue: publickey,password
debug1: Trying private key: /home/xxxxxx/.ssh/id_dsa
debug1: Trying private key: /home/xxxxxx/.ssh/id_ecdsa
debug1: Trying private key: /home/xxxxxx/.ssh/id_ed25519
debug1: Next authentication method: password
yyyyyy@10.0.80.145's password: