Diagnosis
The cup is not aligned with the drive lugs on the power base. Or the locking mechanism inside the base is obstructed.
How to Fix It
Unplug the base. Look inside the motor base. You will see two raised plastic lugs. Align the two notches on the bottom of your cup with those lugs. Press straight down firmly. You should feel a definitive click. Do not twist the cup. It locks with a straight down press.
If That Didn't Work
Food or liquid may have spilled into the base, jamming the locking ring. Unplug, turn base upside down, and tap it out. Clean the lugs and the cup notches with a dry cloth.
Heads Up
Never twist the cup to lock it. This damages the alignment lugs. Always ensure the cup and blade are fully assembled before placing it on the base.