In the "activate" command you're supposed to provide the respective partition of the prepared disk, so probably
ceph-disk activate /dev/sdj1
From the previous output I read that /dev/sdj already had been partitioned in a previous run. If the "activate" command doesn't work I would wipe that disk and start over without partitions on /dev/sdj, running the manual steps for this disk again to see if that changes anything.