Current questions and answers regarding the map editor.

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32MeTpa

heroname definitely needs to be replaced with hero. If that doesn't help and there are no errors in the object names, then it's possible that you can't interact with the monster_arena1_pl1 object, or that you can't attach OBJECT_TOUCH_TRIGGER to arena_level_up_pl1.
Try enabling the console; there are instructions online on how to do it. It will be very helpful.

It seems I have a very rare case.
1) I've already used this command and wrote "heroname" there, and it works, and still does.
2) The console doesn't say anything about my error, either when interacting with the object or afterward.

Added 1 minute ago

32MeTpa

heroname definitely needs to be replaced with hero. If that doesn't help and there are no errors in the object names, then it's possible that you can't interact with the monster_arena1_pl1 object, or that you can't attach OBJECT_TOUCH_TRIGGER to arena_level_up_pl1.
Try enabling the console; there are instructions online on how to do it. It will be very helpful.

I'll try to solve the problem in other ways; if I have any updates, I'll let you know.