Diagnosis
That's the safety interlock doing its job. The NutriBullet isn't sensing that the cup or extractor blade is fully twisted into place on the power base, so it won't engage the motor.
How to Fix It
First, make sure the power base is clean and dry. Then, take your cup (empty is fine) and screw the extractor blade assembly onto it until it's hand-tight—don't force it. Place the assembled cup onto the power base. Now, apply firm downward pressure on the cup while you turn it clockwise. You'll feel a distinct 'click' and the alignment arrows on the cup and base will line up. You should hear the motor start immediately when you press the button. If it doesn't click, unscrew and try again.
If That Didn't Work
The power base itself might have gunk or food residue in the little divot where the blade assembly's drive socket fits. Turn the base upside down and use a dry toothpick or cotton swab to clean out any debris in that central hole. Also, check the plastic drive socket on the blade assembly for cracks or worn teeth.
Heads Up
Never try to run the motor without the cup and blade assembly properly locked in. You can damage the internal gears. The 'click' is non-negotiable—if you don't hear and feel it, the blender won't run.