You must select a delivery method for each item (event or product) that you place in your shopping cart. This can be found just below the seat or product information in the cart to the left of the checkout information (name, card information, etc).
I'm receiving an error message that I must select a delivery method
You'll receive this error message if you have not selected a delivery method for all events in your cart. Delivery methods are specified for each event, so if you are buying tickets to multiple events at the same time, you must select a delivery method for each event in your cart. Any event that does not have a selected delivery method will have those methods listed in red to help draw your attention to them (see picture below).
Once you have selected a delivery method for all events, click "Place Order" again and your order should go through without issue. If you still have issues, please refresh the checkout page and ensure the delivery method is selected before continuing.