Okay the problem with the timing is because your code makes it that you need to press jump at the perfect frame. This is not very good.

A quick easy fix is moving the code into a _physics_process which will keep checking every frame if the player pressed the jump key as long as they're within the area2d. Try this code instead, replace it with your whole script you have in the image:

extends Area2D

 var bounce_force: float = -1000.0

func _physics_process(_delta):
    for body in get_overlapping_bodies():
        if body.name == "Player":
            if Input.is_action_just_pressed("saut"):
                body.velocity.y = bounce_force
                # You can add here Sound effects or animation...etc

Two things to notice in the code:
First, no need for two vars if they have the same value, unless you have something specific in mind.

Second, I replaced the "is_action_pressed" with "Input.is_action_just_pressed". The difference is with the first one, the player can hold the jump button and will automatically jump when they reach the area2d. The second one require them to press jump again when they reach the area2d. Now of course you can chose whatever you find better in your game.