Disabling SIP on a mid 2012 Mac Running Sierra 10.12.16
Instead of just writing "csrutil", replace that with:

/Volumes/Macintosh\ HD/usr/bin/csrutil


so the whole thing becomes:

/Volumes/Macintosh\ HD/usr/bin/csrutil disable


Note that "Macintosh HD" should be replaced with the name of your startup disk. If you do not know the name, then run the following command:

ls /Volumes


It will list the available volumes for you.

If the above gives the error message: "Operation not supported" it means that your recovery system is a version so old that it hasn't got support for the functionality necessary to run csrutil.

In that case try starting up in Internet Recovery mode by holding down Cmd-Opt-R while booting the Mac, which means that the files for the recovery system are downloaded adhoc from Apple's servers. This should give you a version new enough to run csrutil. 

In case that is not possible then you could instead upgrade your own recovery partition to a newer version or alternatively create a recovery partition on a USB pen-drive and boot on that. How to do both is described here:

https://superuser.com/questions/1024493/updating-recovery-partition-in-os-x-el-capitan