You first need to enable Edvoice before being able to send messages. Those schools or users that have not purchased Edvoice will not be able to enable it. In order to enable Edvoice, you first have to select the group where you wish to enable it; once the group has been opened, you have to press on the 3 vertical dots menu on the top right part of the screen. In this menu, if you click on the Edvoice option, you will see the Enable option.
NOTE: You can only enable Edvoice from the Additio App web version.