As @swalkinshaw mentioned, I doubt the template error is related to having staging and production on the same server, and this discourse has a few random unresolved jinja templating threads. Closest thing to a resolution I found was here: “Rebasing the latest [trellis] and doing a provisioning solves the problem of fail2ban.”
I’d make sure your ansible is on the latest (1.9.2 as of this writing) to handle potential jinja templating issues, git pull/rebase the latest trellis, then spin up a brand new server.
I haven’t tried having staging and production on the same server, but I see no reason it wouldn’t work using the setup @swalkinshaw mentioned above.
But for an extra $5/mo at DO, I enjoy having a separate server for staging so I can blow it up, reprovision, and go nuts with it all the time, while leaving my production server untouched. It’s totally worth it to me. I’d recommend separate servers if you’re doing that kind of work with staging.
Of course, there are plenty of use cases in which you wouldn’t need staging on a separate server. In such cases, on the occasion you need to be able to test server setup without disturbing your production site, you could just spin up a temporary disposable testing staging server on the side.