DIAGNOSIS The blade control bail (the lever you hold to start and run the mower) is likely not fully engaged with the engine's flywheel brake. This safety feature locks the engine if the bail isn't depressed.
FIX Ensure the blade control bail is fully pulled back and latched against the handle *before* you pull the starter cord. You should feel and hear a distinct click as it engages. Pull the cord with a smooth, firm motion—don't yank it.
IF THAT DIDN'T WORK The recoil starter assembly itself might be binding. With the spark plug disconnected, manually rotate the blade (using a wrench on the blade bolt) to ensure the engine isn't mechanically seized. If it turns freely, the issue is in the starter mechanism.
HEADS UP Never attempt to bypass or disable the blade control bail safety system. If the mower starts without the bail engaged, the blade will not spin, which is a clear sign the safety interlock is malfunctioning and needs repair.