Ok, the step that couldn't be created is the decision step with the rule 99800005 (I assumed that's the rule you've created according of this thread). The reason is clear: the agent couldn't be retrieved and the rule threw an exception because "Terminate If Rule Resolution Without Result" checkbox was marked.
Get the workflow container values passed to the rule and simulate it through PFAC t-code with those values. It'll give you the details about the problem, and you can figure out what's happening.
