Rosemary, who had been chosen to be the Receiver before Jonas was chosen, failed because she was not able to handle the terrible memories that the Giver had to give her. You can find this out in Chapter 18.
The Giver could not bring himself to give her memories of physical pain like he gave to Jonas, but he felt that he had to give her memories of emotional pain. After a while, she had had enough of that mental anguish and asked to be released. It is not clear if she knew she was going to die.
In the book "The Giver" Rosemary was the giver's natural daughter. She had been chosen to receive the memories. The giver gave her happy memories just like he did Jonas. When Rosemary was ready he began to give her the other kind of memories. He did not start with physical pain like he had with Jonas. Instead, he gave Rosemary emotional pain. He gave her the memory of loneliness.
Rosemary had a lot of difficulty coping with the feeling. At the time the receiver could be released from society. Rosemary could not carry the emotional pain. She had gone to the elders and asked to be released. Release means put to death in the community although the people are not really aware of what it means.
Because of the failure with Rosemary, the people started to get the memories back that she held after her death. It was a disaster. Therefore, the law was changed and the receiver is no longer able to request release.
She died cuz