I can't send Comments to a user.

Check the permissions for that user.

If you are trying to send a comment to another user using the Comment Mode on the Dashboard or the Data Sheet, and you can't see that user in the list of available users, it may be that they have not been given access to the Department or Account/Category that you are working on.

If this is the case, check with your Admin to see if they need to be given that permission, or go to the User page and change their permissions.