I’m not sure if that snippets going to work.
The text that uses the word free! is actually being posted by the ticket price it’s not a heading on the page.
Ah, gotcha. That particular price label is actually being from WooCommerce instead of Event Tickets Plus. So, while the snippet will hit all the instances in Event Tickets Plus, the WooCommerce translation will need to be made in the WooCommerce translation file.
You can find that file here:
/wp-content/plugins/woocommerce/i18n/languages/woocommerce.pot