Hello im using this plugin to make an internal private messagges betwen admin and normal users (Customer)

When i log in as customer, and try to write a message, it says to me:

You cannot message to admin_inicial

In fact, i want the plugin to:

Admin users write to any user
Customers only write to Admin
Customers are not able to write to other customer, so i hide (via css) the field “to” and activate via jQuery the checkbox “send to Admin”
But my customer user cannot send messages

¿There is a setting im doing wrongg?

Thanks