Is it the 2nd bandit base? If that's the one, you're probably missing a core piece which you get by fighting a duo of droids, but you've most certainly defeated before. That duo is an area really close to that base's entrance. I don't recall the correct location now as it's been quite some time I've played the game, but you might have to reenter the area several times for them to spawn. Once you beat them you get the core and you can enter.