there are a few things right off the bat that come to mind. Try moving the shifter around while holding the key on. It could be the brake petal sensor or the neutral safety switch. It could conceivably be your key chip if it has one.
If the machine is still under warranty, take it back to the dealer and have them fix it. That's what the warranty is for, and it's their obligation to do so.
New vehicle warranties come from the manufacturer, not from the dealer. If multiple dealers are available to you, you might take it to another dealer rather than the one whose "fix" was unavailing.