I play swbf online one day and i join a server, but before it logs me on, a message pops up and says: "Cannot join game. You do not have the map currently being played." Well, the maps are not custom maps, they're actual maps that came with the game. Why is it doing this?

This is a classic problem. Since you have more than 25 or so mod maps in your add on folder, which is the maximum SWBF 1.2 can support, TAT3 or Jabba's Palace, is bumped off the list of maps that are correctly loaded. Renaming Addon/TAT3 to !!Tat3 should solve the problem by putting it on the top of the list underneath the Yavin Maps.

I assume that your above message was primarily intended to the "Tatooine : Jabba's Palace" level, Ace', but in case the problems reported and described earlier had been encountered while attempting to play on other maps shipping with Star Wars Battlefront, then I guess that the Instructions and Steps having to be followed might highly differ from the above ones, and that's why I wouldn't mind obtaining additional Informations about your current Star Wars Battlefront Version, pYRo, such as either the latest Update built for it you have installed or the Amount of user-made levels you have downloaded and expanded so far into the "[Hard-Drive's letter]\Program Files\LucasArts\Star Wars Battlefront\GameData\AddOn" Directory...