When opening the invoice, there is no workflow step, and the option to restart the workflow is disabled.
How to resolve this?
You will have to manually assign it to a workflow. If you would like this to be automated, you need to add a default workflow into the Content Type.
If it is already in a workflow, then you may just need to drag up the Workflow tab from below the Content Fields tab.
Is the invoice posted in Kinetic?
How did this document get into DocStar?