It's a TSO message (the IKJ prefix) and means that file ISP06213 wasn't allocated. It is a TSO issue; it definitely is not RACF related or storage routine related. Your TSO session attempted to free up a DD name that was not allocated to it. Since it's an informational message anyway, did the CLIST / REXX complete normally?
I asked the one who was trying to allocate the dataset, to try copying it into a dataset with his UserID as first qualifyer and he managed to do it. So I'm guessing his user doesn't have permission to create or alter datasets with the HLQ that he was trying to modify/create.
Do you think it's still a TSO related issue? I was thinking that maybe RACF is preventing his user to copy that dataset since he doesn't have proper authorization to do it.