You can only send private messages if your task has been assigned. This allows you to exchange contact details with the other Airtasker and arrange finer details about the task.
Private messages are not available before a task has been assigned, however Airtaskers can ask questions and respond in the public comments area of a task. Please do not share any private information in any public areas of the site including any comments or attachments. This is for the purpose of your safety and privacy and to prevent spam.