What was the question and who was asking it? Have you even watched the video you're criticizing her over?
It was a small gaffe because while, yes, what she said was wrong about something every American should know, it came off like somebody tripping and stumbling over their own words or having a brain fart. Everyone has misspoken in their lives and said one thing while meaning something else. You have, too, but unlike Cortez you probably don't have thousands of people hanging on your every public statement ready to pounce and criticize you for any mistake. But just like I'm confident Barack Obama knew exactly how many states were in the U.S. when he said "57 states," I'm confident AOC also does actually know what the branches of government are.
I'd bet the house that she knows more about basic government than you considering it's been a daily part of her life for a couple of years now.