BusyBox `ln -s`: How do I create symlink to a directory on external SD-card?
The rootfs (/) as well as /system have to be mounted with rw permissions.

So, to check for that execute # mount |grep -i rootfs and # mount |grep -i '/system'

If you don't see rw for the permissions, do # mount -o remount,rw / and/or # mount -o remount,rw /system