From the XDA forums, comes a very good tip. If you are having problems patching your Nexus 6P using the SuperSU v2.82 SR3 and you saw that it failed in the logs, Chiyo-chan came up with this tip :-)
I had the same issue.
Looking at the code in the SR2 zip file, I pinpointed it to the file /data/custom_ramdisk_patch.sh. Looking inside the file, it's some remains of Magisk. I removed that file via the TWRP Terminal (rm /data/custom_ramdisk_patch.sh) and it solved the problem.
Hope this helps!"
And there's the quick fix.