What's wrong with "en la mesa"?
Nothing is wrong with that, but if Duolingo always used the easiest response we would never learn the nuances of the language. If you were translating English to Spanish Duolingo should accept 'En la mesa' as a correct answer.
How come not "sobres"? I thought pluralisation applied to adjectives too?
"Sobre" is actually a preposition here, not an adjective, so it doesn't change for plural nouns.
Why not on the desk?
the desk → el escritorio
AGAIN! Duolingo is treating me as if I don't know what "Pasaportes" mean. I've learned that in an earlier "Travel" course!