There are a few reasons this could be happening:
1. Check to see if WhatsApp is installed on your phone. If it's not, you can download it from the app store.
2. Make sure you have the latest version of WhatsApp installed.
3. Open WhatsApp and go to Settings > Chats > Share Chat History and make sure the switch is turned on.
4. If you're still having trouble sharing your chat history, please contact WhatsApp support for further assistance.