No button to reply to a message
If you are in My Messages, viewing a message from another member, and you do not see anywere to type your reply, or a Send button, then your browser isn't displaying the bottom portion of the message that has the text box and Send button.
Please check your browser's zoom setting, and reset it to 100% or less. This should make the whole message window visible on the screen and then you'll see the box to type your message in and the Send button.
If you'd prefer not to change your zoom setting, you can put your cursor within the message window and use your mouse scroll wheel or track pad to scroll down to see the bottom of the window (the scroll bars on the page do not control the message window), where you can type your message and hit Send.