If the 'Email guest' or 'Email agent' button is disabled, this means that there is no email address recorded. You'll need to update the guest's profile.
For guests, you need to click Edit Booking, and then find the room that the guest is in. Next to their name is an Edit button - that'll open up their profile, so you can add in their email address.
For agents, you will need to head to the Agents page in Preno. Find the agent (there's a search bar you can use too), and then click on their name. That'll open up the agent's profile, so you can include their contact information and update their email address.