If appointments have already been booked for the service, you have to cancel the occurence. . Please cancel all associated occurrences with existing bookings in your calendar first. After that, you can delete the service.
Step-by-step guide:
Go to your Admin page.
Go to Services.
Click on the “Edit” button on the group service where you want to delete occurrences.
Scroll to the bottom of the page. Click on the “Delete service” button in the section toolbar.
If there was no booked service, you can click to the Delete service button.