There is no federal law requiring PTO payout. Whether unused PTO is paid when you leave is set by state law and your employer's policy.
PTO payout in the U.S.
How this usually works
PTO payout is governed by state wage law, not federal law. Where a state treats accrued vacation as earned wages, employers must pay it out when you leave. Everywhere else, it comes down to your employer's written policy or contract, and if that policy promises payout, the employer generally has to honor it. A payout is taxed as wages when you receive it. This is general information, not legal advice.