"Hello, my name is Maria."
はじめまして means nice to meet you, not hello. The sentence, "Hello, my name is Maria" is best translated as "こんいちは、マリアと申します。" (Konnichi wa, Maria tomoshimasu). Literal translation is "Hello, Maria is what they call me/Maria is the name" you get the point. It's been a few months since you posted this but Duolingo is accepting こんいちは、マリアと申します。as a correct answer now, but the one with はじめまして is also a possible answer still.
That's because in Japanese they always try to teach foreigners in a way that's both simple and polite. While マルアです "I'm Maria" is correct, for a Japanese person on some occasions it might be improper or a bit childish to introduce oneself like this. マリアともうします "My name is Maria/You can call me Maria" is a lot more formal, so it is way better for introductions (when you first meet someone you always have to be formal unless they are a small child).
i thought that はじめましてマリアです would be correct, but what does the ともうします mean?
ともうします, as far as I understand it (I'm still learning myself) is a polite and humble way of saying "I am called ...". This is fairly common in Japan when introducing yourself for the first time to someone of higher status, or someone you want to lower your status and show humility towards (like one of your customers). あたしはマリアです is technically also a correct way of introducing yourself, but would be considered very casual.
私の名前は〜です。 [わたしのなまえは〜です]。= My name is.....
〜と申します。[〜ともうします] ＝ You may call me.... / They call me....
Hello, my name is Maria. = 今日は、私の名前はマリアです。 [こんにちは、わたしのなまえはマリアです。]
Hello, I'm Maria. = 今日は、私はマリアです。 [こんにちは、わたしはマリアです。]
Hello, (I'm) Maria. = 今日は、マリアです。 [こんにちは、マリアです。
Hello can be こんにちは or sometimes 初めまして [はじめまして] - Usually translated as "nice to meet you", but literally "beginning" so used for the first time you meet someone.
Yet another question where says I'm wrong (and I was) but gives a solution that is not possible. It asks me to say "マリアです" but there is no "です" to select. Here's a screenshot: https://i.gyazo.com/7eccd1fdabf9039f55f591331e8c04dd.png