If you are seeing a (!) or exclamation mark prior to your template name when trying to send communication to a client, contact third party or referrer then this simply indicates that the template includes appointment merge fields. These merge fields can only be used for appointment reminders (either automatic or manual) and cannot be used for other communication.
If you send the template regardless then these fields will appear as blank spaces in the email, letter or SMS.
To get around this limitation you can use next appointment merge fields, which reference the next appointment for that client in the system.