Any 'On' condition (with the green arrow next to it) is a trigger. Triggers are called whenever the event occurs OUTSIDE of the regular script loop, so you can't place any code after them expecting that code to be called next, it won't be.
So what is happening here is the key is pressed, the JumpKeyPress is set to true and the Score set to 100, then the code returns. It does not keep going. So the next time the script runs, your Every tick event sets JumpKeyPress to false, and the Is JumpKeyPress won't be evaluated.
What you could do here is not reset every tick, but have the Is JumpKeyPress reset the value.
KeyDown works because it is a regular event, not a trigger.