You can only leave a message to your coach whilst using the Shot Scope dashboard https://dashboard.shotscope.com/.
The comments tab will only be available on certain pages. On the pages where comments is enabled there will be a button at the bottom right of the page labelled 'Comments'. Click this button to open the comments feature.
Once the comments feature is open you will see a list of coaches to choose from. Select the correct coach, type in your message and then hit the enter key on your keyboard. The comment will then be sent to your coach.