Voucher programs currently only exist in a few areas. Cleveland, Milwaukee and Florida come to mind.
Each area has its own different rules, but in general your eligibility depends on your income and how good the schools in your area are.
Take the (hypothetical) little Rastas for example: Springfield (IL) public schools are atrocious, but there are worse schools in Illinois and so I don’t think they’d be considered “failing” by any statewide standard. Also, my income isn’t what one would conisder “low” (though it’s not high by any standard, either), so I probably wouldn’t be eligible for any voucher if Illinois ever offered one.
YMMV