I agree with what has been said above and its the exact same issue i had with mine about 15 minutes into my first run. Im guessing the little screw with holds the pinion to the motor shaft has come loose.
What i recommend you do which is the same thing i did after this happened is to take the tuck inside and start pulling parts apart and putting lock-tite where required.
The truck is going to probably continue to fall apart unless this is done. I know it sucks to have to do this since you just bought it and cant wait to rip it up with it, but its something you must do by choice or something you will have to do anyway and have to purchase the parts that come loose and you will then have to order them.
Another important thing i did rite away when i heard it was tighten the slipper clutch all the way. Im lucky i caught this in time and am sure many who dont have a little experience with just keep on driving and fry their slipper.
Its recommended to do this with any RTR you purchase especially one like the Savage Flux which has so much torque and power out of that brushless system it comes with.
Besides that its an amazing truck and one of my top 3 ever purchased off roaders.