The bit is not inserted correctly or the chuck jaws are obstructed.
- Remove the battery.
- Open the chuck jaws by turning the sleeve counterclockwise.
- For a drill bit: Let the bit strike the bottom of the chuck. Center it, then lift it about 1/16" off the bottom.
- For a screwdriver bit: Insert it far enough for the chuck jaws to grip the hex portion.
- Close the chuck by turning the sleeve clockwise. Tighten until you hear a ratcheting sound and the sleeve won't turn further.
If that didn't work, check for debris in the chuck jaws. Clean them out. The ratcheting sound during tightening is normal; it's part of the locking feature.
Heads up: Only use accessories recommended for this tool. Others may not seat properly and can be hazardous.