workflow in AX 2009

Hello Friends,

Actually I am working on Purchase Requisition workflow in AX 2009. Workflow configure Successfully.

But when we create Purchase requisition and submit it is correct after that when we open the Action button → View History

on this form status is workflow Stopped(Error) and on line ->tab->Tracking Details Workflow stop error -"Position not found"

Actually i have already assign the position to employee.

but really i am not completely understand the process i.e. how and when to start the workflow and when which user is login

please help me about Purchase requisition workflow from staring level(user assign role)

please help me …