I'm having the similar problem with this missions.
I have achieved the destruction of the shields of all 6 fortresses. The next step is to see Lieutenant Marklo to "Start [HEROIC 2+] Flashpoint: The Star Fortress". Instead, I get Start Story Mode, Start Veteran Mode and Refuse Mission as options.
Some of you said Veteran Mode is [HEROIC 2+] but it's not true because the game said that I was having 2 different levels for the same mission so I should choose one. There must be something wrong with the game or maybe I'm suppose to do something that is not mentioned.