You will need to add the number of tickets to the price type you would like in the Quantity box prior to clicking "Add to Cart". 

If no price types are showing on your web form, follow these steps:
  1. Go to the Web tab and click Manage Event Registration Forms
  2. Find the event, click the drop-down arrows next to it and click Options
  3. On this window, check the boxes in the "Include" column for the price types you want available for this event
  4. Save
If the boxes were already checked for those prices types, use these steps:
  1. Go to the Tickets tab and click Sales Methods
  2. Click Online Sales, then select the Price Types tab
  3. Find the applicable price type on the list to make sure that it has a green check-mark next to it
  4. If it doesn't, click the drop-down arrows next to the price type and click "Allow price type"
Once the price types have been enabled for online sale for your event, customers should be able to click "Add to Cart" without seeing the error.