The most common causes of this are either that your user account does not have an email address associated with it, or that the supplier or customer that you are trying to send the email to does not have an email address registered in their record. To check the email address on your user record, please follow the below steps:
Ensure you are logged into Dimensions as a user with access to the system control area.
Select System Control
Select User records
Select the relevant user account
Select Email in the top ribbon
Ensure there is an email specified
If that is filled out, then you will need to check the address on the customer or supplier record (as this may be missing or incorrectly entered). Please see our article on email address for customers and suppliers.