In ipcs -s (semaphores) or ipcs -m (shared memory) you’ll see an entry for the Natural Bufferpool,
in this case they key is 0x61111111, you would look for what the messages says - shmkey = 13111111
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x61111111 262151 sag 660 3145728 0
You see the owner (here: sag) and the permission (here: perms = 660) for the shared memory segment.
What you should check now is if the user receiving the message has sufficient rights to access the bufferpool.
Now, if you are sure this user should be able to access the bufferpool, check if this user tries to start Natural pointing to a bufferpool different to what’s active.
natparm -> Configuration -> Local Configuration File -> Buffer Pool Assignments
will show you the defined ones, ipcs will show what’s active.