Why am I getting the "This service is an add-on" message?

If you're getting the "This service is an add-on" message on your client booking pages, this happens when there is no duration set on the service, so the software is treating the service as an "add-on" to another service.

To fix this, go to Setup > Services, click on the service and add a time duration in the field to the right of the price (e.g. enter "60m" for 60 minutes).