If I was laid off due to hour being cuts but on my separation letter my manager wrote down that I was fired due to not showing up for work and insubordination, is there anything I can do legally?

The letter is by itself irrelevant--it's a letter to you and, not to overemphasize the point, but it could say you were laid off because you're a Martian and it would be equally irrelevant. As long as it's just in the separation letter, it does not matter: simply do not show the letter to anyone if you disagee with it. You also can't legally force them to change the letter so long as its only shared with you.
BUT there are two ways in which a false statement of why you were let go *could* hurt you, and if either of those occur, you can take action.
1) If you apply for unemployment insurance and they try to deny on the false grounds that you were terminated for cause. If they do that, first you can appeal a denial (if you are denied) both within the labor dept./unemployment agency and, if necessary, to a court.
2) If they tell anyone else (anyone other than you) a factual falsehood (i.e. an actual lie, not merely an opinon) that damages your reputation or causes you economic harm, you can sue them for defamation. This includes lying if anyone prospective employer calls for a recommendation. A personal injury attorney (the same kind of lawyer who handles slip-and-fall or auto accident cases) could help you with this.