That's the problem - there's no option to select the heroic version being presented by the Sergeant.
I'm having this exact same problem, with both the Tatooine and the Voss quests on two different toons. I do the preliminary solo stuff, take down the shield bunker with the local resistance contact, talk to him afterwards - he says he's going to Odessen and my mission log updates to tell me to talk with the Sergeant guy to complete the heroic +2 destroy the fortress mission. HOWEVER, when I go back to talk with the Sergeant, he does NOT give me a choice of selecting the heroic version - just the solo mission again, in either story or verteran mode. I went ahead and tried the story one again on one of the toons, but sure enough it was simply a replay of the original solo version, and ended with my log still telling me to talk with the Sergeant to do the heroic version.
So, what is the trick to getting the Sergeant to offer the heroic version so we can at least try it?
[EDIT] after reading some more posts I found about this problem, is it just that they renamed the "Heroic" mission to "Veteran" on the Sergeant's menu? I stayed away from selecting that one because in some other context (replaying the Chapters themselves) the choice of "Veteran" was just the same play-though as "Story" but at a much harder difficulty (and when I tried it on Chapter 1 my lvl 70, 228-geared toon was routinely getting slaughtered by the first solo silver sky trooper I encountered on Darth Marr's ship). I guess I'll give it a try and see if selecting "Veteran" does the trick here.
[EDIT] OK, yes, that was the key - choosing "Veteran" does actually start you on the different, "heroic" path with the buffs from your specialists and hidden tools/extra buffs/powers from them scattered around (and unlike that Chapter veteran setting, I'm not getting slaughtered by the trash in this run). Would have been nice if they had gotten their terminology straight - if they were going to change it on the Sergeant's menu they should have changed it in the mission log and related text as well.