How do I compete this mission?

[Factabulous]

Success!!!!
I've figured it. You don't only have to scroll to collect units but you also have to click the scroll button AGAIN to confirm which is why I kept flying off with nothing. Not very user friendly especially how long it takes to even fly from one place to another

Nope. Sounds like you still haven't quite figured it out. Ah well, hopefully one day you will read one of the many replies that told you how it works, though I'm feeling less hopeful