At T+0, the flight computer ignites the engines, but the vehicle doesn't liftoff yet. Instead, it is held down for two seconds while the flight computer verifies that all engines are working correctly (to prevent blowing up both the vehicle and everything in its surroundings).

It seems that the checks failed here and the flight computer aborted launch at T+00:01.

commentator said that the flight computer canceled launch because the readings from the 9 merlin engines were "not nominal"