Couple of things:
The question mark in the code shouldn't be there. Was that for my benefit? When I run it without that question mark the only error in allegro is a note that 176 is an invalid call for ComboF[] (change the for loop to:
for(int i = 0; i < 176; i++)
The other issues with this are:
The Magic Compass item should be a Custom Itemclass and it MUST be an Equipment Item. If Equipment Item is not checked, the item disappears as soon as you collect it.
The script only runs when you enter the screen, so if Link never changes screens it won't run and the combos don't change.
For the FFC itself, on the Flags tab you can check the box marked Run Script at Screen Init. If it's checked, the script will run before the screen is visible and the player will never see the flagged trees. If you leave it unchecked, the player will see the trees while the screen scrolls and then they'll disappear.
https://ruq.io/MagicCompass.qst