Short answer: There’s no federal penalty; the ACA individual-mandate penalty has been $0 since 2019. But a handful of states impose their own penalty if you go without coverage.
The federal tax penalty for being uninsured was reduced to $0 starting in 2019, so most Americans face no federal fine. However, several states reinstated their own individual mandates with penalties: California, Massachusetts, New Jersey, Rhode Island, and the District of Columbia enforce a financial penalty on residents who go without coverage (Vermont mandates coverage but does not impose a penalty). Penalty amounts vary, often a flat per-person figure or a percentage of income, whichever is greater. If you live in one of those states, having qualifying coverage (or an exemption) avoids the state penalty.