The maven release plugin assumes it is running in an environment that has permission to write to the remote repository. That message likely means that it does not have permission to write to the repository.
A message somewhat like that is reported by the maven release plugin when it is unable to push changes to the remote repository.
You might try a small experiment from inside the job to push something harmless to the remote repository. For example, you could create a tag from inside the job, push the tag, and then remove the tag. If you can’t push a tag or remove a tag, the maven release plugin can’t do the job it needs to do.
Yes, I agree that the ssh private key needed to push to the repository is not available from the agent that is attempting to perform that push. Try what I suggested earlier to see if that helps you identify the location that needs the private key.
sounds like you don’t have the ssh agent plugin installed.
The problem is your agent doesn’t have the proper ssh key to talk to your repo. You can add the key to the home directory of the user that is running the agent. Or you can use sshagent and jenkins to manage it in memory so keys are not on disk.