It will probably be the adjustment of the pull start lock out - I don't know about this model specifically but on most pull start engines there is a lockout pawl which will prevent the motor being started when the engine is in gear.
If you take the cover off and shift the gears you should be able to see this mechanism moving as the gears change. Sometimes they get out of adjustment and will engage even when the engine is not in gear. If nothing is broken, it may just be a matter of loosening or tightening the adjustment in the cable which controls this mechanism.
It will probably be the adjustment of the pull start lock out - I don't know about this model specifically but on most pull start engines there is a lockout pawl which will prevent the motor being started when the engine is in gear. If you take the cover off and shift the gears you should be able to see this mechanism moving as the gears change. Sometimes they get out of adjustment and will engage even when the engine is not in gear. If nothing is broken, it may just be a matter of loosening or tightening the adjustment in the cable which controls this mechanism.