Hi kimco,
The translation string for Add to Cart was only recently added to the translation file for WooCommerce Tickets. I suspect that the language you are using has not yet been updated to include it. If your language wasn’t updated in yesterday’s 3.5 release, I’d love to have you update the translation for WooCommerce Tickets, including the Add to Cart string. If you’re interested, check out the Getting Started post in our Translations Forum where you’ll find the instructions and files you need.
Best,
Leah