Have you updated your iPhone to the latest iOS? If not, please review the support article linked below and update your device. After updating, please do a forced restart. A forced restart is more than turning your device off and on. Please follow the instructions in the article linked below to insure your iPhone is force restarted. Lastly, try adding your tickets again.
I do want to caution you, that there are multiple posts across several social media sites with people posting about this issue. It does not appear to be an issue, but rather a TicketMaster issue, or an issue with the third party you have purchased the tickets from.
Have you contacted TicketMaster support?
Update iOS on iPhone - Apple Support
Force restart iPhone - Apple Support