Rooted Nexus 6 Android N NPD35K Issues

I've been trying to root my Nexus to get access to PS4 remote play and add the cast feature for my Samsung TV. I've managed to root it and checked that I'm rooted using root checker and terminal.

However, having rooted I still cannot edit anything whatsoever. I need to edit build.prop for the screen casting but can't save it, only read. I don't know if there is a master read/write permission for root somewhere? Still pretty inexperienced with rooting.

Would really appreciate if someone could help, I'm pulling my hair out!