"The girl does not love us."
Translation:Flickan älskar inte oss.
Yes, your sentence is correct too.
The normal placement of a sentence adverbial (here "inte") is after the subject and the verb but before the object: "Flickan älskar inte oss".
But if the object is a pronoun that is not stressed, the adverbial can be placed after the object: "Flickan älskar oss inte".
This means that oss is stressed in the default translation but not in yours.
Very strange. As you can see, Flickan älskar inte oss is the main translation so it should always be accepted. Maybe a temporary glitch, a bug, or you had some other typo without noticing. If this happens again to you or anyone else, it would be great if you could share a screenshot.
ej is a more formal word for inte. It means exactly the same, but it's mainly used on signs etc. It's good to know the word, but you don't really need to use it yourself. Any accepted answer can be shown to you depending on what you input though, so you might see it in corrections or as 'another correct answer'.