If it is caused by System Integrity Protection, you can boot into recovery by holding down command-R during startup. You probably will have to enter a password. Once you are in the recovery screen, go to the Terminal in the Utilities menu.
In the terminal enter:
csrutil disable
Hit return and you should see a message that system integrity is disabled. Then restart. But don't forget to go back and enable it again later. It is an important security feature.