It is not possible to finish the first mission. I've beaten the legions near the burning tree, but nothing is happing after. I can see the ship that I guess is supposed to pick me up. I can even stand on it but nothing works...
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Once you beat the legions near the tree, you'll need to proceed to top right (as indicated in your radar), past the sweeper bot in the hallway, open the door, meet Ikora, kill more Cabal... FYI plenty of tutorials and walkthroughs online...