I assume, this is a riding lawn tractor. One likely problems is the seat switch.

The switch is normally closed - to ground out the ignition. When someone is sitting in the seat, the switch opens. When the blades are engaged, another safety switch connects a circuit to the seat switch. If the seat switch is defective, the engine will shut off.

Yes this is a riding mower and brand new. Where is the seat switch?

Maybe look under the seat?????

It could be that your deck where the blade are is full of grass. That happens to me all the time. Get rid of the grass underneath...especially if its wet.