there could be many things that could cause this, however i do not remember an event "item placed in slot one" instead try to copy the exact trigger as it is in your map.
event: Advanced - Any unit moves any item to inventory slot 1 condition: Advanced - Any unit moves any item to inventory slot 1
action: Special Effect - Destroy specialeffect[(Player number of (Owner of (Triggering unit)))] Special Effect - Create a special effect attached to the weapon of (Triggering unit) using war3mapImported\Konstrukt_AssaultRifleAttachment.MDX Wait 0.05 seconds Set specialeffect[(Player number of (Owner of (Triggering unit)))] = (Last created special effect)
Why is it that the event & condition are the same?
I think the whole trigger will not work simply because you dont have the correct condition. Besides you have the action; Special Effect - Create a special effect attached to the weapon of (Triggering unit) using..etc.. & therefore you should have the corresponding condition for this.