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.
Duo te voy a arrancar todas las plumillas de tu sucio trasero, me haces equivocar por cualquier tontería...!