Of course Buddhists can commit suicide. They can also kill, steal, harm people, speak divisively, speak abusively, tell lies, be greedy, hate people and all the rest. The teaching is that doing any of those things is a misguided attempt to reach happiness. If someone tells lies or acts out of greed, it’s because he thinks it will lead to happiness. One way to look at it is that someone who commits suicide thinks that there is no hope for happiness at all, and death will at least stop the pain. What the suicide doesn’t understand from this point of view is that he will be reincarnated, and will have to deal with clearing the negative karma which brought him pain in the next life. Another way to look at it is that someone who wants to kill himself doesn’t understand that the circumstances of this life aren’t really that important; that with a deeper understanding of what reality really is, even the most terrible conditions can be experienced as happiness. People who commit suicide deserve sincere compassion.