https://m.reddit.com/r/nexus6/comments/3rkniy/busybox_wont_install_on_60/ I had the same issue initially. It means that busybox wasn't able to write to the system partition. As other posts have mentioned, you'll need to install a custom kernel for full root access. SuperSU installed manually or by WUGS will give you root privileges, but the stock kernel in 6.0 uses SELinux (an additional security later) to prevent even root from writing to the system partition.