This error normally comes from the transaction trying to post at the same time as another and use the same number.
Best thing to do is to delete the batch if the system lets you if it doesn't then end task through the Task Manager.
If you select the Task Manager and then click on OpenEdge Graphical Client (32bit), select End Task.
Once this has closed then re-open Proclaim and delete any stray batches that are sat at drafted.
Select General Postings and then create the posting again.