SuperSU binaries won't install to my HTC One M8
I have had this issue and there are 3 ways to try to get around this.

You will need a custom recovery to attempt 1. or you can use Odin alternatively for 2. below. The superSU binaries are often (but not always) installed via recovery and the app itself gives this option.


When superSU asks you to update the binaries, rather than select 'Normal' select 'Recovery' and see if it can install that way.
Flash superSU.zip directly in your recovery or Odin. Here is the link to Chainfire's latest version. I have tried this myself and it fixed the issue for me on a number of devices. 
Install an alternative super user app from the market. (This can be messy, and you need to ensure superSU is completely removed)