jenkins is a service account, it doesn’t have a shell by design. It is generally accepted that service accounts shouldn’t be able to log in interactively.
I didn’t answer this one initially as it’s a duplicate of a question that has been moved to server fault. I should have answered rather than linked to the answer in a comment.
if for some reason you want to login as jenkins, you can do so with:
sudo su -s /bin/bash jenkins