There should be two 'Aid' items down at the bottom of the list (unlocked with the default tech you begin the mission with). You may need to scroll down if you haven't pruned the list by building many of the special 'one per galaxy' buildings across your planets.
If for some reason those aren't there, another workaround is to destroy a building on Ambassador and rebuild it to get around the inability to end turn. I finished that mission a couple of days ago on the most recent patch (with most planets build queues completed) and for me the trick was making sure to scroll down the build list to get to the Aid items.
Hope that helps, and happy campaigning!
