Some states require event organizers to collect tax on their events.  You can set up sales tax on your event when creating it or even after it's on sale.

During Event Set Up

Sales tax can be added on the Publish page during the event set up process.

Click on Customize Sales Tax Fees to enable the fee.

Enter your state's sales tax percentage and where you would like to apply it. Scroll back to the top of the page and click Save.