It's likely that you have know goodbye in Chinese is 拜拜(bái bái), and it also could be 再见(zài jiàn) formally, but are you sure you pronounce goodbye in Mandarin accurately? Don't worry, Lilian teacher from Hanbrige Mandarin is here to help!

Detailed Guideline: http://www.hanbridgemandarin.com/article/daily-chinese-learning-tips/hello-goodbye-in-chinese/