You may need to setup an SSH keys or environment variables to your Jenkins installations. One of the easy way to do this is to setup those environment variables inside the jenkins user account. If you installed Jenkins in CentOS or Ubuntu via YUM or APT; These package managers will setup a jenkins user account without login capability and shell. That means you cannot simply SSH to it. Using your other Linux account you can login as jenkins using the command snippet below.
sudo su -s /bin/bash jenkins
What this command does is it set the shell for the target user in this case /bin/bash.