Can I cancel a single step in a Workflow? To go back to the previous step as Active?

I don't think so. Netsuite already thinks that step is complete.

You just want to go back to the previous stage in your workflow? I do that by adding a button to the record that says "Go Back" and then execute a transition to the previous stage upon clicking that button. You can make the button only available to Administrator's too if you want more control over who can click it

Interesting, thanks @AI1