NVRAM: Error clearing firmware variables: (iokit/common) not permitted
Solution 1:
Despite the error message, the NVRAM has been cleared. Found this on another forum that proves it out:
$ sudo nvram TestVar=HelloWorld
$ sudo nvram -p | grep 'TestVar'
TestVar HelloWorld
$ sudo nvram -c
nvram: Error clearing firmware variables: (iokit/common) not permitted
$ sudo nvram -p | grep 'TestVar'
$