Usually when that happens it's because you picked up the weekly Ascendant Challenge bounty last week, then carried it over into this week. Each bounty is tied to a specific location, so if you do this week's challenge with last week's bounty, it won't complete. Try deleting your Ascendant Challenge bounty, and picking up the one Petra has in her inventory, and then doing the challenge again. If this is what happened, the text on Petra's bounty now, should be different from the one you have in your inventory. Also, the Toland quote on the bounty gives a hint as to where that week's challenge is located. It will typically contain a word that is also in the name of that week's challenge location.