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 (pronounced “suh-van-uh”) is a noun in American English. 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 (including Canada, Australia, and former British colonies, it is spelled “savannah.”
- Areas like this in the US are typically called prairies, and they are found in Plains states Nebraska, Kansas, Iowa, and Oklahoma.
Savannah spelled with an initial capital letter, 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, a state in the US southeast. Savannah is famous for its historic 1800s architecture, its public squares, and an elegant Riverwalk. Coming in by boat, Savannah might look like a savanna on first look, but its not land, instead its lots of marsh grass growing in salty, thick mud. A person couldn’t easily get out and walk on it, they would get mired.
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.
ORIGINALLY PUBLISHED DECEMBER 10, 2015.
Kathleen W Curry 
Hi— I’m working in Toronto with others on a manuscript that describes plants and vegetation of Ontario. We have limited grasslands, and both spellings to which you refer are in use here. You might want to look at https://grammarist.com/spelling/savanna-savannah/ where the distinction is made between an American spelling (savanna) and the usage elsewhere in the world (savannah), in both cases for the grasslands with scattered trees. The OED (accessecd online through my university) definitely recognizes savannah, but I was unable to find the portion of the entry dealing with the U.S. usage.