Is it possible you already have one flagged that you've yet to complete? Check your /keyitem list. Also:

Quote Originally Posted by Fynlar View Post
This is not the case anymore. Outpost supply runs you have done now travel with you between nations.
Outposts you've unlocked in Bastok would still have to be unlocked again after changing to, for example, San d'Oria. What S-E changed was that now, if you go back to Bastok again, the system will remember your previously-unlocked outposts, rather than making you do them again.