When you are trying to make up a word that doesn't exist, the grammatically correct way to solve the problem is to re-arrange the sentence so that you can use the "base" form of the word you are trying to turn into a new word. I think that was his point - the grammatical rule of adding suffixes to words to give them new meaning is not a universal rule that you can apply to any word you like. Rather, consider it the science behind how the words became that way in the first place. Meaning, you are being taught it so you can have a mental image of why the words mean what they mean, not so you can use it to create new words that don't exist. You never should have put "Tupac's Autonomousity", you should have put "Would you label Tupac 'autonomous'", or better yet, "Describe Tupac's ego in one word".
