There could be a few reasons why no time slots are shown in your booking widget. Here are some:
All assigned personnel are not available.
To remedy this, configure your personnel's work schedules appropriately.
All assigned resources are not available (this only applies if your service has at least 1 assigned resource).
To remedy this, configure your resource(s)' availability appropriately (it's configured the same way as personnel work schedules).
No personnel is assigned to your service (applies only if your service's 'Time selection mode' is set to "Display time slots with automatic dispatch" or "Display time slots with automatic dispatch (resource)")
All personnel assigned to your service have "Enable online booking" disabled (applies only if your service's 'Time selection mode' is set to "Display time slots with automatic dispatch" or "Display time slots with automatic dispatch (resource)")
See here to learn more about time slots and related settings.