DIAGNOSIS The locking mechanism is obstructed by a spent capsule that didn't eject properly, or the mechanism's plastic latch is broken.
FIX Do not force the lever. Unplug the machine. Place one hand firmly on top of the closed lid. With your other hand, firmly and quickly smack the side of the machine near the brew head. This vibration can dislodge a stuck capsule. Then try the lever again.
IF THAT DIDN'T WORK A piece of a damaged capsule is physically jamming the mechanism. You may need to manually intervene. While pressing the lever as far as it will go, use a thin, blunt tool (like a butter knife) to gently pry the lid open from the front edge. Be prepared for debris to fall out.
HEADS UP Never brew without a capsule. This can cause the system to over-spin and damage the motor. If the lever feels loose or clicks without resistance, the internal plastic latch has snapped. This requires a full brew head assembly replacement.