Occasionally, you may click the Reply link on a conversation and end up on the 'Notes' panel instead. this article provides information on this issue.
This issue is usually a sign that the brand to which the conversation belongs doesn't have an email address set up for it yet. Once you've added an address for that brand, you'll be able to reply to the conversation like normal.
For help, check out the user guide article about adding email accounts to your Kayako.