Apple - csrutil: command not found

The netboot image loaded by booting to Internet Recovery Mode apparently doesn't contain the executable csrutil.

The OS X Base System loaded while booting to Recovery Mode should contain it though. By pressing cmdR a 2-step procedure is initiated: First the Mac is booted to Recovery HD and then after expanding BaseSystem.dmg to "OS X Base System" (which is the name of the mounted BaseSystem.dmg). If you see an animated globe after pressing cmdR your Mac probably doesn't have a Recovery HD.

Check the system version (or product version) of the Recovery HD/Base System:

To check the system version (1 in the screenshot below), boot to your main volume and enter the following in Terminal:

diskutil list #to get the disk identifier of your Recovery HD; usually it's disk0s3 with a size of ~650 MB
diskutil mount disk0s3
cat /Volumes/Recovery\ HD/com.apple.recovery.boot/SystemVersion.plist | grep -A 2 ProductVersion

Additionally you may mount BaseSystem.dmg and check the system version (2 in the screenshot below) there also:

open /Volumes/Recovery\ HD/com.apple.recovery.boot/BaseSystem.dmg
cat /Volumes/OS\ X\ Base\ System/System/Library/CoreServices/SystemVersion.plist | grep -A 2 ProductVersion

Download and reinstall the latest OS X El Capitan full installer if the system version of the base system is 10.10.x or lower.

Late to the party but had the same problem. Solved it by, at startup, hold down the option key, ⌥, until you can choose what to boot from and then click on the recovery one, should be Recovery-"version". Then you can follow the same steps as earlier stated - open terminal and write csrutil disable/enable.