As Jmanis said, these tourist sightseeing missions usually have more than one location that the NPC wants to visit, so it's possible that you missed that, and perhaps went to the final destination first, hence why you got the message saying return to base but were not able to complete the mission.
The mission description will always tell you what destinations the tourist wants to visit, and in which order, and from what I remember you must visit each location in the order it's listed in the mission description.
Also, Speedcuffs point is useful. If you ever have a mission that doesn't seem to be able to complete, look at its description in the transactions tab (left HUD) as if there's something stopping it completing that will be displayed, usually in red.