Diagnosis
Incorrect assembly. The bail (safety start lever) is likely misaligned with its pivot point or the lower handle bracket is interfering.
How to Fix It
Revisit assembly steps 5 and 6. Loosen the handle knobs. Ensure the bail assembly's metal rods are correctly seated in the pivot points on the mower's upper body. The lower part of the handle must be correctly aligned so it does not block the bail's full range of motion. Tighten the handle knobs only after confirming the bail moves freely from released to fully depressed.
If That Didn't Work
The interlock switch connected to the bail may be defective or misaligned. This requires inspection of the switch mechanism inside the housing.
Heads Up
The mower will not operate unless the bail is fully depressed and held against the handle. This is a critical safety feature.