TranslatePress does not detect the {site_title} tag itself as a translatable string. This is a dynamic placeholder used by WooCommerce in combination, but it’s not part of the actual string TranslatePress detects.
Try searching for the full text of the email string, but without the placeholder tags like {site_title}, {order_id}, etc. TranslatePress captures the static parts of the string, not the dynamic tags or the output they render.
First be sure the email is sent in order for the string to be detected. Also, check in the Regular tab as well since it’s a user-driven string added manually by you.