The Refer a Friend system is currently down, you need to wait until it is fixed before you can refer your friend.
More information: http://www.swtor.com/community/showthread.php?t=853848
Hey folks,
During maintenance we discovered an issue with our referral program. We have gone ahead and temporarily disabled it until we can address it. If you already have someone referred, you will still receive your rewards normally during this time. However, while it is disabled, you cannot refer any new players. Your referral links will not work during this time.
Once we have a timeframe on getting things resolved we will pass it on. Thanks everyone.
-eric
