Check the journal on the target machine closely for sshd errors
root@server# useradd -m foob root@server# journalctl -fu sshd sshd: Invalid user foob from x.x.x.x sshd: User foob not allowed because account is locked sshd: input_userauth_request: invalid user newuser [preauth] foob@client$ ssh foob@server
In my case, the account is locked because the user has no password. Create a new password for the user and try again.
root@server# passwd foob foob@client$ ssh foob@server