You have to understand few things. Disenchanct is point type spell meaning that you can't order unit to cast it on destructible but first set position of target tree and then call function that orders dummy to disenchant that location.
You are forced to use abilities that target trees by default (there are few exceptions surely) because channel can't target tree ;/
It's great that you use Naish Sentinel spell - because it works properly for this kind of actions.
Additionaly:
- your triggers leaks
- the string order is 'dispel' not 'disenchant'
- you have to consider speed of missle, if it's instant that you can run actions instantly, but if it has some speed (thus there is delay before owl will actually sit on tree) TriggerSleepAction or other 'delay' stuff should be introducted to your trigger.
Next time please add stuff required to test the trigger (trees) and create proper dummy unit =)