If I was a General Manager for a restaurant and was fired for insubordination, can they withhold my final paycheck?

No they can not.  Under Iowa law a final paycheck must be given the employee on or before the next scheduled pay date. There are no circumstances under which an employer can totally withhold a final paycheck under Iowa law.  Start by filing a complaint with the Department of Labor.  If that does not work then consider a court action against them.  Good luck.