The Rancher doc says in the automatic upgrades section:

Danger

If the K3s cluster was imported into Rancher, Rancher will manage the system-upgrade-controller deployment and plans. Do not follow the steps on this page.

I imported my k3s cluster and it appears in Rancher, but the system-upgrade controller is not running in k3s. And there is no option to upgrade k3s in the Rancher cluster management dashboard. Any idea why this would happen?

It needs to be on a version supported by Rancher to be upgradeable

Ahh, ok. It is v1.24.10+k3s1. For some reason that is not supported by rancher, I guess.