I am trying to send an email but when I press ‘Send’ the system tells me that some people cannot be sent the message?

This error message will display 'Some' or 'None' depending on the recipient list.

You have selected the ‘Send to primary contacts only’ box and the children you have selected to send the email to do not have a primary contact. Go to ‘Users’ and then select ‘List’ from the sub menu and you will be able to search for the user in question. Once you find their record, you can put a tick in the ‘Primary contact’ box for the parent. Alternatively, you can remove the option to send to primary contacts and the email will send to all contacts assigned to the students.