You should just use the standard list of accepted countries that is used for literally every other geography-related Jetpunk quiz. And Vatican City is on that list.
Also, it's frustrating that countries like "St. Kitts and Nevis" can only be spelled one exact way in your quiz. Usually there are certain deviations and shortcuts that are acceptable, too, such as just typing "st kitts".
Neither "Saint Kitts and Nevis" nor "Saint Vincent and the Grenadines" nor "Saint Lucia" work. Please accept the full names, not only the abbreviations!
Also, it's frustrating that countries like "St. Kitts and Nevis" can only be spelled one exact way in your quiz. Usually there are certain deviations and shortcuts that are acceptable, too, such as just typing "st kitts".