Capacity of a service is related the product inventory. Your customer can’t book an appointment if the product is out of stock. 

That’s why we recommend you should enable Continue selling when out of stock.

Case 1: The product (service) has variants

1. From Shopify admin page, click Products

2. Click to a product service

3. Select all variants

4. Click “More actions“

5. Select “Continue selling when out of stock”

6. Click Save

Case 2: The product (service) has no variants

1. From Shopify admin page, click Products

2. Click to a product service

4. Check on the checkbox Continue selling when out of stock

5. Click Save