Only new copies of the game can be used for export.
The purpose of an export is to allow a player to condense all of their owned Rock Band music library into one game. Each game is only licensed to be exported once and has a one-time use export code to make this possible. If you've purchased a used copy of the game, chances are the previous owner has already used the code to export their songs. In that case, you won't be able to use that code again for export.
Export codes can't be purchased on their own, so if you would like to export the music from your game, you will need a brand new copy.