Try this:
+ Sprite2: is on ground
-> Sprite2: Set 'canUseJetpack' to 0
+ Sprite2: Pick by Sprite2[Platform].VectorY Greater than 500
+ Sprite2: is falling
-> Sprite2: Set 'canUseJetpack' to 1
+ Sprite2: Value 'canUseJetpack' Equal to 1
+ MouseKeyboard: Key Z is down
-> Sprite2: Set vertical speed to Sprite2[Platform].VectorY-6000*timedelta