In OFP square brackets like this [ ] normally specify an array. An array is a group of things, you can think of it as a kind of list. Obviously an array may contain only one item, or several.
Some commands require arrays as arguments, others don't. When you get this error message it means you have an array somewhere instead of an object. In this case, remove the [] thusly
guy1 action ["dropweapon","M16","guy1"]
and see how you get on.
This is really a completely different question from the original one, so you could have started a new thread. Not that it really matters.