From the message guide:
PDSMAN is already active in another address space. As only one copy of PDSMAN can be active at any time, this startup is bypassed.
Receiving this message when no other PDSMAN address space is actually active indicates that a previous PDSMAN termination was incomplete. PDSMAN can be started with a PARM=CLEAR to complete the previous termination and then restarted as normal.
This message and the PARM=CLEAR is only documented in the PDSM Message Guide
It is also recommended that this option be used in the PROC used to start PDSM if you have frequent shutdowns or the system has severe performance issues and PDSM is required to be recycled.