Does the assignment you are trying to open still exist? You said "I have open B-1 case and completed BFLOW flow" (emphasis mine). If you completed the flow, does that mean B-1 is now in a resolved state? If so, there is no assignment to reopen. You need to get the work object back into the flow and onto an assignment shape. This will create a NEW assignment for that work object. Depending on how complex the flow for B-1 is, this could be a simple task or something requiring lots of logic to determine which assignment in the flow you care about, how the reopen impacts SLAs, etc.