I guess you are using the ResumeFlow to move your current assignment forward within the flow and reach another task (assignment).
But you get the error message described on this discussion when executing ResumeFlow. The meaning is, your ResumeFlow cannot be executed because there is no more assignment to resume. Somebody or something else has already moved this assignment to the next step.
So you need to find who or what has worked your assignment you wanted to resume. Maybe case history could help you understanding.