"La fille est seule."
Adjectives match the noun's gender and number. In this case, "fille" is feminine, so you must use the feminine form "seule".
When I typed "La fille est seul" without the e at the end I started to panic because I thought it was going to be wrong. But then duolingo still accepted my answer. Hmm, is my answer still grammatically correct even if I forgot the e at the end?
No, it's because You made only one mistake. Duolingo let You make one mistake, but is always the information that You almost wrote the sentence right.