Didn't get too much from the video, but I would check the battery voltage first.
Cranking sounded slow, but not odd to me.
Most likely you need to at least charge the battery if you haven't.
Make sure the gas is fresh and the tank is clean inside. There should be an inline fuel filter inside the fuel line where it attaches to the carb that you should check and replace if necessary.
Carbs may not be completely clean. There's quite a bit of disassembly and details required to really get them 100% clean. If you want it done professionally, talk to ducatiman.
You may want to make fine adjustments to the idle mixture, and then the idle speed, once you are sure everything else is correct.
This section has a lot of info -
https://faq.ninja250.org/wiki/Ninja250_Howto