You had a few extra and misplaced brackets; also, the function isn't SetEnemyActivate, it's SetEntityActive. Here's the revised script:
////////////////////////////
// Run first time starting map
void OnStart()
{
AddEntityCollideCallback("Player", "Area", "Aparecemonstruo", true, 0);
}
void Aparecemonstruo(string &in asParent, string &in asChild, int alState)
{
SetEntityActive("enemigo", true);
ShowEnemyPlayerPosition("enemigo");
}
////////////////////////////
// Run when entering map
void OnEnter()
{
}
////////////////////////////
// Run when leaving map
void OnLeave()
{
}
I hope that helped
I rate it 3 memes.