First, you will want to verify you have an Email Template set up for a Service Reminder. Go to Marketing > Email Templates.

To create a new template, scroll to the bottom and fill out the Template Name and Subject. Add whatever text, images, merge tags, etc. that you would like. Our example is below. Scroll to the bottom and click Create Template. 

Once you have an Email Template set up, go to Marketing > Automations. Click on + New Automation. 

Name the Automation whatever you'd like. Continue to select the desired drop-down options for Trigger Condition, Run Time, and Action. Then select the Email Template you built for Service Reminders.

For the example above, this Automation is set up to Email the Customer the selected Email Template, 1 Day Before the Event Happens. Event meaning the service or visit. 

There are various options you can choose for this automation such as to Text Customer or Email Company Owner. Or to send the reminder 2 or 3 Days Before The Event Happens.