Translation:The girl puts the book down on the table.
It might sound unnatural, but keep in mind this course is for learning Norwegian, not about learning English. I guess most students aren't native English speakers, but just use English because their primary language isn't available as 'basic' language in Duolingo. So Duolingo should be a tad more forgiving for errors in English than it is for errors in Norwegian.
You should be able to translate
legger as either "lay(s)" or "put(s)," because these words all require a direct object (e.g., "boken").
However, "lie(s)" is not an acceptable translation because "lie(s)" is intransitive and cannot take a direct object.
In other words, you actively lay (
legger) something on a surface:
Jeg legger et brev på bordet= I lay a letter on the table.
Objects and people passively rest and lie (
ligger) on a surface:
Brevet ligger på bordet= The letter lies on the table.