"I work thirty-one hours."
Asking that shows you're doing a good job looking for the particle は in a sentence. In this case however, there is no particle. The verb for "to work" is 働く (はたらく) or in this case はたらきます, the は is just the first syllable of it, and is read in the normal way (ha) instead of as a particle (wa).
You could explicitly state the topic, e.g., by prepending わたしは to the sentence, but in this case it's implied. If you responded to a question asking how long someone else worked then the implied topic would be that person.
There's nothing special about the verb はたらく, it just happens to begin with a は.
In order to type in kanji, you need to have downloaded the Japanese characters onto your computer or phone in order to do so. I recommend following this guide, it includes the steps on how to go about that process step by step with pictures, whether you're using windows, mac, an iphone or an android: https://www.tofugu.com/japanese/how-to-install-japanese-keyboard/
三十一時間働きます, with kanji for "hatarakimasu", is also marked wrong. I get that people will not have learned the kanji for "hataraku" yet, but when you're typing with a Japanese keyboard, the default is to convert to kanji and not leave things in kana, so Duolingo should accept kanji too.
For the sake of learning, I think the dictionary form is important, too, which happens to be the informal form. When learning indogermanic languages, you still learn the infinitive form even when you seldom use it directly. But it helps you to get how to build the other verb forms.
The long vowel mark ー only coincidentally looks like 一 (the number 1) in the sans-serif font that Duolingo uses. It's a different character (and, when writing vertically, the long vowel mark is a vertical line). On a Japanese keyboard, type the number 1 and you should see 一 in the options right under the Western "1".