"No, I am a girl."
Translation:Non, je suis une fille.
Nouns in French are either feminine or masculine. "Fille" feminine noun, so we use "une" in front of it. Masculine nouns take "un."
A human teacher may recognize that as a typo, but unfortunately the computer grader will count that as a different word "file" which means "line."
"J'suis" may be commonly said in casual speech, but it isn't correct to write it. It should be "je suis."
They both mean "a or an," but un is used with masculine nouns and une with feminine ones.
Ex. un garçon, un vélo, un stylo (a boy, a bike, a pen)
une fille, une table, une pomme (a girl, a table, an apple)