你可以尝试一下 nexus root toolkit,保留数据刷机，然后重新刷 recovery,再刷 supersu.

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.

谢谢各位，是 SELinux 的原因，刷个 kernel 把 SElinux 从 enfocing 改成 permissive 就可以了。

1.很多第一条就是本科或以上；
2.祝你好运，顺利找到实习，积累项目经验。