What should you check next if you already replaced the battery and the starter and your vehicle still will not start?

Since you changed your battery and starter I'm assuming you are getting fuel and spark is your problem. You should check your starter relay, the wires from the relay to your starter, the wires from your battery to the relay, the wires from your locking cylinder to the relay, and your locking cylinder. You might also check your alternator. You can take it to a parts store to have it bench tested for free.