1) the conditionals would have no choice but to make this invisible due to you having all possible values of score turning this invisible (one of them oughta be "true")
2) the _visible property has an underscore preceding it
3) to assign a value to something you need to use = To compare things for equality you use ==
4) are you sure "this" is what you intend to target? I cannot tell from the code due to not knowing what timeline it occupies, but "this" represents the timeline that the code is in... so you made need to use the button's instance name instead since buttons do not normally have a timeline that you can put code like that in...
if (score < 200){
this._visible = false;
}
else if (score >=200){
this._visible = true;
}