Error "File Master IMS could not allocate temporary DDN IMS" is given for a very specific situation, related to the presence of one or more GSAM PCB's in the PSB.
File Master is trying to allocate a temporary load library with this command:
TSO ALLOCATE FI(IMS) DA('userid.SYSFMxxx.TEMPIMS') UNIT(SYSDA) CYL SPACE(5 3) DSORG(PO) DSNTYPE(LIBRARY)
userid is the userid of the person executing File Master
xxx is a 3-digit random number
In the above command, some parameters are derived from the File Master CDBILOAD. E.g. "CYL" can be "TRK" and DSNTYPE(LIBRARY) will be replaced with DIR(5) if CDBILOAD is a PDS.