Ciao Giacomo,
Thanks for using our plugins and welcome to the forums!
I’m sorry about this translation issue. I can help you fix this.
It looks like that some of the .po / .mo files haven’t been updated to the latest standing, so the original source string doesn’t exist in the \event-tickets\lang\event-tickets-it_IT.po file.
To fix this you can use either PoEdit or the Loco Translate plugin.
Open the \event-tickets\lang\event-tickets-it_IT.po file with PoEdit and go to Catalog > Update from POT file … option, and choose \event-tickets\lang\event-tickets.pot as a source. This should update the string set in the .po file and you should be able to find “Buy Now!”
Translate, save and upload the .po and .mo files to your server, preferably under wp-content/languages/plugins so your translations are kept even after a plugin update.
If you are using Loco Translate, then open your translation file there and hit the Sync button. And then save.
Let me know if this helps you fix it.
Cheers,
Andras