Savanna and savannah are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Savanna is a noun. It means large stretches of grasslands with few trees. Savannas are typically found in in the eastern parts of the African continent. [In British English, it is spelled “savannah.”Thanks Stuart Otway for pointing this out.]
Savannah is a proper noun. Proper nouns are always capitalized, because they identify a specific person, place, or thing. Savannah is a city in coastal Georgia, in the USA. Savannah is famous for its historic 1800s architecture, its public squares, and an elegant Riverwalk.
The following story uses both words correctly:
Sadie hailed from Savannah, Georgia, but she dreamed of places thousands of miles away. She hoped to one day photograph the large, exotic animals found on African savannas.