If you search the available cultures using the following code, you will find that the code you're looking for isn't in the set of available cultures.
var cultures = CultureInfo.GetCultures(CultureTypes.AllCultures).Where(c => c.Name.Equals("Lt-sr-SP"));
If you search using
var culture = CultureInfo.GetCultures(CultureTypes.AllCultures).Where(c => c.EnglishName.Contains("Serbian"));
You can see that the culture code you're looking for is sr-Latn-RS.