I took a quick look and already found a mistake.
In the QuestionBoxForPlayers function, you define a function that will be called when the "Yes" answer is given. This function does not accept any parameters, i.e., in the line function captureF (heroName), the parentheses should be empty.
There is a solution, for example, like this: before this entire block, define a global variable, i.e., write a line like hero=''.
Then, in the function where QuestionBox is written, assign the hero's name to this variable like this: hero= heroName. And then use this variable in the StartCombat function. It will look something like this:

dir = GetMapDataPath()
hero='';
Trigger(OBJECT_TOUCH_TRIGGER, "tres1", "tres1F");
function tres1F (heroName)
		number=GetCurrentPlayer();
                      hero=heroName;
		QuestionBoxForPlayers(number,dir.."Capture.txt", "captureF", "nocaptureF");
end;
function captureF ()
	number=GetCurrentPlayer();
	MessageBoxForPlayers(number,dir.."gettreasure.txt");
	StartCombat (hero, nil, 2,
CREATURE_THANE, 20,
CREATURE_WARLORD, 20, nil);
end;
function alreadyCapturedF (heroName) 
	number=GetCurrentPlayer();
	MessageBoxForPlayers(number,dir.."alreadyCaptured.txt");
end;
function nocaptureF ()
	number=GetCurrentPlayer();
           hero='';
	MessageBoxForPlayers(number,dir.."Coward.txt");
end;
SetObjectEnabled("tres1", nil);

I'd also like to note that it would be good to add a function to the StartCombat function that will be called after the battle. Thanks to it, you can reward the hero, remove the trigger from the building, etc.

Yes, and MessageBoxForPlayers takes not the player as the first parameter, but its filter GetPlayerFilter(number). There will be no visible difference if there are 1-2 players, but errors will occur for >2. It is also desirable to add a check to see if the player is a computer or not.